Saturday, July 10, 2021

Top 5 Core Java Exception Handling interview questions and answers

What is Exception Handling?

Exception Handling is a mechanism to handle runtime errors. It is mainly used to handle checked exceptions.

1.) What is the difference between Checked Exception and Unchecked Exception?

1)Checked Exception:

The classes that extend the Throwable class except RuntimeException and Error are known as checked exceptions e.g.IOException, SQLException, etc. Checked exceptions are checked at compile-time.

2)Unchecked Exception:

The classes that extend RuntimeException are known as unchecked exceptions e.g. ArithmeticException, NullPointerException, etc. Unchecked exceptions are not checked at compile-time.

2.) What is the base class for Error and Exception?

Throwable.

3) Is it necessary that each try block must be followed by a catch block?

It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method.

4.) What is finally block?

finally block is a block that is always executed.

5.) Can finally block be used without catch?

Yes, by try block. finally must be followed by either try or catch.

Full Stack Java Developer Community

Full Stack Java Developer Community is all about upgrading your Skills on different technologies in the real world. We give Tutorials on Java, J2EE, Spring, Hibernate, SQL, Reactive, Microservices, Docker, Kubernetes, and many more. We also make real-time projects to get you started on different technologies and provide tech support for problems you face in real-time. It's a Tab of technology where we get you started to script your ideas.

What are Packages in core java and how to use them ?

A Java package is a technique for organizing Java classes into namespaces similar to the modules of Modula, providing modular programming in Java. Java packages can be stored in compressed files called JAR files, allowing classes to be downloaded faster as groups rather than individually.

Packages are used in Java in order to prevent naming conflicts, to control access, to make searching/locating and usage of classes, interfaces, enumerations and annotations easier, etc.

A Package can be defined as a grouping of related types(classes, interfaces, enumerations and annotations ) providing access protection and namespace management.

Some of the existing packages in Java are::

· java. lang - bundles the fundamental classes

· java.io - classes for input, output functions are bundled in this package

Programmers can define their own packages to bundle groups of classes/interfaces, etc. It is a good practice to group related classes implemented by you so that a programmer can easily determine that the classes, interfaces, enumerations, annotations are related.

Since the package creates a new namespace there won't be any name conflicts with names in other packages. Using packages, it is easier to provide access control and it is also easier to locate the related classes.

Creating a package:

When creating a package, you should choose a name for the package and put a package statement with that name at the top of every source file that contains the classes, interfaces, enumerations, and annotation types that you want to include in the package.

The package statement should be the first line in the source file. There can be only one package statement in each source file, and it applies to all types in the file.

If a package statement is not used then the class, interfaces, enumerations, and annotation types will be put into an unnamed package.

What are Packages in java and how to use them?

OOPS CONCEPT

Packages in Java are a mechanism to encapsulate a group of classes, interfaces, and sub-packages. Many implementations of Java use a hierarchical file system to manage source and class files. It is easy to organize class files into packages. All we need to do is put related class files in the same directory, give the directory a name that relates to the purpose of the classes, and add a line to the top of each class file that declares the package name, which is the same as the directory name where they reside.

In java, there are already many predefined packages that we use while programming.

For example: java.lang, java.io, java.util etc.

However one of the most useful features of java is that we can define our own packages

Advantages of using a package:

Before discussing how to use them Let see why we should use packages.

§ Reusability: Reusability of code is one of the most important requirements in the software industry. Reusability saves time, effort and also ensures consistency. A class once developed can be reused by any number of programs wishing to incorporate the class in that particular program.

§ Easy to locate the files.

§ In a real-life situation, there may arise scenarios where we need to define files of the same name. This may lead to “name-space collisions”. Packages are a way of avoiding “name-space collisions”.

Types of packages:

1) User-defined package: The package we create is called a user-defined package.
2) Built-in package: The already defined package like java.io.*, java.lang.* etc are known as built-in packages.

Defining a Package:

This statement should be used at the beginning of the program to include that program in that particular package.

package <package name>;

Full Stack Java Developer Community

Basics of Spring Framework

This spring tutorial provides in-depth concepts of Spring Framework with simplified examples. It was developed by Rod Johnson in 2003. Spring framework makes the easy development of the JavaEE application.

Spring Framework

Spring is a lightweight framework. It can be thought of as a framework of frameworks because it provides support to various frameworks such as Struts, Hibernate, Tapestry, EJB, JSF, etc. The framework, in a broader sense, can be defined as a structure where we find solutions of various technical problems.

The Spring framework comprises several modules such as IOC, AOP, DAO, Context, ORM, WEB MVC, etc. We will learn these modules on the next page. Let's understand the IOC and Dependency Injection first.

Inversion Of Control (IOC) and Dependency Injection

These are the design patterns that are used to remove dependency from the programming code. They make the code easier to test and maintain. Let's understand this with the following code:

class Employee{

Address address;

Employee(){

address=new Address();

}

In such a case, there is the dependency between the Employee and Address (tight coupling). In the Inversion of Control scenario, we do this something like this:

class Employee{

Address address;

Employee(Address address){

this.address=address;

}

Thus, IOC makes the code loosely coupled. In such a case, there is no need to modify the code if our logic is moved to a new environment.

In the Spring framework, the IOC container is responsible to inject the dependency. We provide metadata to the IOC container either by XML file or annotation.

Advantage of Dependency Injection:

makes the code loosely coupled so easy to maintain

makes the code easy to test

Advantages of Spring Framework:

There are many advantages of the Spring Framework. They are as follows:

1) Predefined Templates-

Spring framework provides templates for JDBC, Hibernate, JPA, etc. technologies. So there is no need to write too much code. It hides the basic steps of these technologies.

Let's take the example of JdbcTemplate, you don't need to write the code for exception handling, creating connections, creating statements, committing transactions, closing connections, etc. You need to write the code for executing the query only. Thus, it saves a lot of JDBC code.

2) Loose Coupling-

The Spring applications are loosely coupled because of dependency injection.

3) Easy to test-

The Dependency Injection makes it easier to test the application. The EJB or Struts application requires the server to run the application but the Spring framework doesn't require a server.

4) Lightweight-

Spring framework is lightweight because of its POJO implementation. The Spring Framework doesn't force the programmer to inherit any class or implement any interface. That is why it is said non-invasive.

5) Fast Development-

The Dependency Injection feature of Spring Framework and it support to various frameworks make the easy development of the JavaEE application.

6) Powerful abstraction-

It provides a powerful abstraction to JavaEE specifications such as JMS, JDBC, JPA, and JTA.

7) Declarative support-

It provides declarative support for caching, validation, transactions, and formatting.

Full Stack Java Developer Community

SQL(Structured Query Language) Tutorials

Introduction to SQL

SQL is a standard language for accessing and manipulating databases.

What is SQL?

SQL stands for Structured Query Language
SQL lets you access and manipulates databases
SQL is an ANSI (American National Standards Institute) standard

What Can SQL do?

SQL can execute queries against a database
SQL can retrieve data from a database
SQL can insert records in a database
SQL can update records in a database
SQL can delete records from a database
SQL can create new databases
SQL can create new tables in a database
SQL can create stored procedures in a database
SQL can create views in a database
SQL can set permissions on tables, procedures, and views

SQL is a Standard - BUT...

Although SQL is an ANSI (American National Standards Institute) standard, there are different versions of the SQL language.

However, to be compliant with the ANSI standard, they all support at least the major commands (such as SELECT, UPDATE, DELETE, INSERT, WHERE) in a similar manner.

Note: Most of the SQL database programs also have their own proprietary extensions in addition to the SQL standard!

Using SQL in Your Web Site

To build a website that shows data from a database, you will need:

An RDBMS database program (i.e. MS Access, SQL Server, MySQL)
To use a server-side scripting language, like PHP or ASP
To use SQL to get the data you want
To use HTML / CSS

RDBMS

RDBMS stands for Relational Database Management System.

RDBMS is the basis for SQL, and for all modern database systems such as MS SQL Server, IBM DB2, Oracle, MySQL, and Microsoft Access.

The data in RDBMS is stored in database objects called tables.

A table is a collection of related data entries and it consists of columns and rows.

Database Tables

A database most often contains one or more tables. Each table is identified by a name (e.g. "Customers" or "Orders"). Tables contain records (rows) with data.

In this tutorial, we will use the well-known Northwind sample database (included in MS Access and MS SQL Server).

Below is a selection from the "Customers" table:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin	12209	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constitución 2222	México D.F.	05021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	05023	Mexico

The table above contains five records (one for each customer) and seven columns (CustomerID, CustomerName, ContactName, Address, City, PostalCode, and Country).

SQL Statements

Most of the actions you need to perform on a database are done with SQL statements.

The following SQL statement selects all the records in the "Customers" table:

Example

SELECT * FROM Customers;

In this tutorial, we will teach you all about the different SQL statements.

Keep in Mind That...

SQL is NOT case sensitive: SELECT is the same as select

The semicolon after SQL Statements?

Some database systems require the semicolon at the end of each SQL statement.

The semicolon is the standard way to separate each SQL statement in database systems that allow more than one SQL statement to be executed in the same call to the server.

In this tutorial, we will use the semicolon at the end of each SQL statement.

Some of The Most Important SQL Commands

SELECT - extracts data from a database
UPDATE - updates data in a database
DELETE - deletes data from a database
INSERT INTO - inserts new data into a database
CREATE DATABASE - creates a new database
ALTER DATABASE - modifies a database
CREATE TABLE - creates a new table
ALTER TABLE - modifies a table
DROP TABLE - deletes a table
CREATE INDEX - creates an index (search key)
DROP INDEX - deletes an index

SQL SELECT Statement

The SELECT statement is used to select data from a database.

The SQL SELECT Statement

The SELECT statement is used to select data from a database.

The result is stored in a result table, called the result-set.

SQL SELECT Syntax

SELECT column_name,column_name
FROM table_name;

and

SELECT * FROM table_name;

Demo Database

In this tutorial, we will use the well-known Northwind sample database.

Below is a selection from the "Customers" table:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin	12209	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constitución 2222	México D.F.	05021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	05023	Mexico
4	Around the Horn	Thomas Hardy	120 Hanover Sq.	London	WA1 1DP	UK
5	Berglunds snabbköp	Christina Berglund	Berguvsvägen 8	Luleå	S-958 22	Sweden

SELECT Column Example

The following SQL statement selects the "CustomerName" and "City" columns from the "Customers" table:

Example

SELECT CustomerName, City FROM Customers;

SELECT * Example

The following SQL statement selects all the columns from the "Customers" table:

Example

SELECT * FROM Customers;

SQL SELECT DISTINCT Statement

The SELECT DISTINCT statement is used to return only distinct (different) values.

The SQL SELECT DISTINCT Statement

In a table, a column may contain many duplicate values; and sometimes you only want to list the different (distinct) values.

The DISTINCT keyword can be used to return only distinct (different) values.

SQL SELECT DISTINCT Syntax

SELECT DISTINCT column_name,column_name
FROM table_name;

Demo Database

In this tutorial, we will use the well-known Northwind sample database.

Below is a selection from the "Customers" table:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin	12209	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constitución 2222	México D.F.	05021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	05023	Mexico
4	Around the Horn	Thomas Hardy	120 Hanover Sq.	London	WA1 1DP	UK
5	Berglunds snabbköp	Christina Berglund	Berguvsvägen 8	Luleå	S-958 22	Sweden

SELECT DISTINCT Example

The following SQL statement selects only the distinct values from the "City" columns from the "Customers" table:

Example

SELECT DISTINCT City FROM Customers;

SQL WHERE Claus

The WHERE clause is used to filter records.

The SQL WHERE Clause

The WHERE clause is used to extract only those records that fulfill a specified criterion.

SQL WHERE Syntax

SELECT column_name,column_name
FROM table_name
WHERE column_name operator value;

Demo Database

In this tutorial, we will use the well-known Northwind sample database.

Below is a selection from the "Customers" table:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin	12209	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constitución 2222	México D.F.	05021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	05023	Mexico
4	Around the Horn	Thomas Hardy	120 Hanover Sq.	London	WA1 1DP	UK
5	Berglunds snabbköp	Christina Berglund	Berguvsvägen 8	Luleå	S-958 22	Sweden

WHERE Clause Example

The following SQL statement selects all the customers from the country "Mexico", in the "Customers" table:

Example

SELECT * FROM Customers
WHERE Country='Mexico';

Text Fields vs. Numeric Fields

SQL requires single quotes around text values (most database systems will also, allow double quotes).

However, numeric fields should not be enclosed in quotes:

Example

SELECT * FROM Customers
WHERE CustomerID=1;

Operators in The WHERE Clause

The following operators can be used in the WHERE clause:

Operator	Description
=	Equal
<>	Not equal. Note: In some versions of SQL this the operator may be written as !=
>	Greater than
<	Less than
>=	Greater than or equal
<=	Less than or equal
BETWEEN	Between an inclusive range
LIKE	Search for a pattern
IN	To specify multiple possible values for a column

SQL AND & OR Operators

The AND & OR operators are used to filter records based on more than one condition.

The SQL AND & OR Operators

The AND operator displays a record if both the first condition AND the second condition are true.

The OR operator displays a record if either the first condition OR the second condition is true.

Demo Database

In this tutorial, we will use the well-known Northwind sample database.

Below is a selection from the "Customers" table:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin	12209	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constitución 2222	México D.F.	05021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	05023	Mexico
4	Around the Horn	Thomas Hardy	120 Hanover Sq.	London	WA1 1DP	UK
5	Berglunds snabbköp	Christina Berglund	Berguvsvägen 8	Luleå	S-958 22	Sweden

AND Operator Example

The following SQL statement selects all customers from the country "Germany" AND the city "Berlin", in the "Customers" table:

Example

SELECT * FROM Customers
WHERE Country='Germany'
AND City='Berlin';

OR Operator Example

The following SQL statement selects all customers from the city "Berlin" OR "München", in the "Customers" table:

Example

SELECT * FROM Customers
WHERE City='Berlin'
OR City='München';

Combining AND & OR

You can also combine AND and OR (use parenthesis to form complex expressions).

The following SQL statement selects all customers from the country "Germany" AND the city must be equal to "Berlin" OR "München", in the "Customers" table:

Example

SELECT * FROM Customers
WHERE Country='Germany'
AND (City='Berlin' OR City='München');

SQL ORDER BY Keyword

The ORDER BY keyword is used to sort the result-set.

The SQL ORDER BY Keyword

The ORDER BY keyword is used to sort the result-set by one or more columns.

The ORDER BY keyword sorts the records in ascending order by default. To sort the records in descending order, you can use the DESC keyword.

SQL ORDER BY Syntax

SELECT column_name,column_name
FROM table_name
ORDER BY column_name,column_name ASC|DESC;

Demo Database

In this tutorial, we will use the well-known Northwind sample database.

Below is a selection from the "Customers" table:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin	12209	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constitución 2222	México D.F.	05021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	05023	Mexico
4	Around the Horn	Thomas Hardy	120 Hanover Sq.	London	WA1 1DP	UK
5	Berglunds snabbköp	Christina Berglund	Berguvsvägen 8	Luleå	S-958 22	Sweden

ORDER BY Example

The following SQL statement selects all customers from the "Customers" table, sorted by the "Country" column:

Example

SELECT * FROM Customers
ORDER BY Country;

ORDER BY DESC Example

The following SQL statement selects all customers from the "Customers" table, sorted DESCENDING by the "Country" column:

Example

SELECT * FROM Customers
ORDER BY Country DESC;

ORDER BY Several Columns Example

The following SQL statement selects all customers from the "Customers" table, sorted by the "Country" and the "CustomerName" column:

Example

SELECT * FROM Customers
ORDER BY Country, CustomerName;

SQL INSERT INTO Statement

The INSERT INTO statement is used to insert new records in a table.

The SQL INSERT INTO Statement

The INSERT INTO statement is used to insert new records in a table.

SQL INSERT INTO Syntax

It is possible to write the INSERT INTO statement in two forms.

The first form does not specify the column names where the data will be inserted, only their values:

INSERT INTO table_name
VALUES (value1,value2,value3,...);

The second form specifies both the column names and the values to be inserted:

INSERT INTO table_name (column1,column2,column3,...)
VALUES (value1,value2,value3,...);

Demo Database

In this tutorial, we will use the well-known Northwind sample database.

Below is a selection from the "Customers" table:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
87	Wardian Herkku	Pirkko Koskitalo	Trikatu 38	Oulu	90110	Finland
88	Wellington Importadora	Paula Parente	Rua do Mercado, 12	Resende	08737-363	Brazil
89	White Clover Markets	Karl Jablonski	305 - 14th Ave. S. Suite 3B	Seattle	98128	USA
90	Wilman Kala	Matti Karttunen	Keskuskatu 45	Helsinki	21240	Finland
91	Wolski	Zbyszek	ul. Filtra 68	Walla	01-012	Poland

INSERT INTO Example

Assume we wish to insert a new row in the "Customers" table.

We can use the following SQL statement (without specifying column names):

INSERT INTO Customers
VALUES ('Cardinal','Tom B. Erichsen','Skagen 21','Stavanger','4006','Norway');

or this SQL statement (including column names):

INSERT INTO Customers (CustomerName, ContactName, Address, City, PostalCode, Country)
VALUES ('Cardinal','Tom B. Erichsen','Skagen 21','Stavanger','4006','Norway');

The selection from the "Customers" table will now look like this:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
87	Wardian Herkku	Pirkko Koskitalo	Trikatu 38	Oulu	90110	Finland

Did you notice that we did not insert any number into the CustomerID field?
The CustomerID column is an AutoNumber field and is automatically updated with a unique number for each record in the table.

AutoNumber is a type of data used in Microsoft Access tables to generate an automatically incremented numeric counter. The default AutoNumber type has a start value of 1 and an increment of 1.

Insert Data Only in Specified Columns

It is also possible to only insert data in specific columns.

The following SQL statement will insert a new row, but only insert data in the "CustomerName", "City", and "Country" columns (and the CustomerID field will of course also be updated automatically):

INSERT INTO Customers (CustomerName, City, Country)
VALUES ('Cardinal', 'Stavanger', 'Norway');

The selection from the "Customers" table will now look like this:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
87	Wardian Herkku	Pirkko Koskitalo	Trikatu 38	Oulu	90110	Finland
88	Wellington Importadora	Paula Parente	Rua do Mercado, 12	Resende	08737-363	Brazil
89	White Clover Markets	Karl Jablonski	305 - 14th Ave. S. Suite 3B	Seattle	98128	USA
90	Wilman Kala	Matti Karttunen	Keskuskatu 45	Helsinki	21240	Finland
91	Wolski	Zbyszek	ul. Filtra 68	Walla	01-012	Poland
92	Cardinal			Stavanger		Norway

SQL UPDATE Statement

The UPDATE statement is used to update records in a table.

The SQL UPDATE Statement

The UPDATE statement is used to update existing records in a table.

SQL UPDATE Syntax

UPDATE table_name
SET column1=value1,column2=value2,...
WHERE some_column=some_value;

Notice the WHERE clause in the SQL UPDATE statement!
The WHERE clause specifies which record or records that should be updated. If you omit the WHERE clause, all records will be updated!

Demo Database

In this tutorial, we will use the well-known Northwind sample database.

Below is a selection from the "Customers" table:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin	12209	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constitución 2222	México D.F.	05021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	05023	Mexico
4	Around the Horn	Thomas Hardy	120 Hanover Sq.	London	WA1 1DP	UK
5	Berglunds snabbköp	Christina Berglund	Berguvsvägen 8	Luleå	S-958 22	Sweden

SQL UPDATE Example

Assume we wish to update the customer "Alfreds Futterkiste" with a new contact person and city.

We use the following SQL statement:

UPDATE Customers
SET ContactName='Alfred Schmidt', City='Hamburg'
WHERE CustomerName='Alfreds Futterkiste';

The selection from the "Customers" table will now look like this:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Alfred Schmidt	Obere Str. 57	Hamburg	12209	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constitución 2222	México D.F.	05021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	05023	Mexico
4	Around the Horn	Thomas Hardy	120 Hanover Sq.	London	WA1 1DP	UK
5	Berglunds snabbköp	Christina Berglund	Berguvsvägen 8	Luleå	S-958 22	Sweden

Update Warning!

Be careful when updating records. If we had omitted the WHERE clause in the the example above, like this:

UPDATE Customers
SET ContactName='Alfred Schmidt', City='Hamburg';

The "Customers" table would have looked like this:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Alfred Schmidt	Obere Str. 57	Hamburg	12209	Germany
2	Ana Trujillo Emparedados y helados	Alfred Schmidt	Avda. de la Constitución 2222	Hamburg	05021	Mexico
3	Antonio Moreno Taquería	Alfred Schmidt	Mataderos 2312	Hamburg	05023	Mexico
4	Around the Horn	Alfred Schmidt	120 Hanover Sq.	Hamburg	WA1 1DP	UK
5	Berglunds snabbköp	Alfred Schmidt	Berguvsvägen 8	Hamburg	S-958 22	Sweden

SQL DELETE Statement

The DELETE statement is used to delete records in a table.

The SQL DELETE Statement

The DELETE statement is used to delete rows in a table.

SQL DELETE Syntax

DELETE FROM table_name
WHERE some_column=some_value;

Notice the WHERE clause in the SQL DELETE statement!
The WHERE clause specifies which record or records that should be deleted. If you omit the WHERE clause, all records will be deleted!

Demo Database

In this tutorial, we will use the well-known Northwind sample database.

Below is a selection from the "Customers" table:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin	12209	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constitución 2222	México D.F.	05021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	05023	Mexico
4	Around the Horn	Thomas Hardy	120 Hanover Sq.	London	WA1 1DP	UK
5	Berglunds snabbköp	Christina Berglund	Berguvsvägen 8	Luleå	S-958 22	Sweden

SQL DELETE Example

Assume we wish to delete the customer "Alfreds Futterkiste" from the "Customers" table.

We use the following SQL statement:

DELETE FROM Customers
WHERE CustomerName='Alfreds Futterkiste' AND ContactName='Maria Anders';

The "Customers" table will now look like this:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constitución 2222	México D.F.	05021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	05023	Mexico
4	Around the Horn	Thomas Hardy	120 Hanover Sq.	London	WA1 1DP	UK
5	Berglunds snabbköp	Christina Berglund	Berguvsvägen 8	Luleå	S-958 22	Sweden

Delete All Data

It is possible to delete all rows in a table without deleting the table. This means that the table structure, attributes, and indexes will be intact:

DELETE FROM table_name;

or

DELETE * FROM table_name;

Note: Be very careful when deleting records. You cannot undo this statement!

SQL SELECT TOP Clause

The SQL SELECT TOP Clause

The SELECT TOP clause is used to specify the number of records to return.

The SELECT TOP clause can be very useful on large tables with thousands of records. Returning a large number of records can impact performance.

Note: Not all database systems support the SELECT TOP clause.

SQL Server / MS Access Syntax

SELECT TOP number|percent column_name(s)
FROM table_name;

SQL SELECT TOP Equivalent in MySQL and Oracle

MySQL Syntax

SELECT column_name(s)
FROM table_name
LIMIT number;

Example

SELECT *
FROM Persons
LIMIT 5;

Oracle Syntax

SELECT column_name(s)
FROM table_name
WHERE ROWNUM <= number;

Example

SELECT *
FROM Persons
WHERE ROWNUM <=5;

Demo Database

In this tutorial, we will use the well-known Northwind sample database.

Below is a selection from the "Customers" table:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin	12209	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constitución 2222	México D.F.	05021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	05023	Mexico
4	Around the Horn	Thomas Hardy	120 Hanover Sq.	London	WA1 1DP	UK
5	Berglunds snabbköp	Christina Berglund	Berguvsvägen 8	Luleå	S-958 22	Sweden

SQL SELECT TOP Example

The following SQL statement selects the two first records from the "Customers" table:

Example

SELECT TOP 2 * FROM Customers;

SQL SELECT TOP PERCENT Example

The following SQL statement selects the first 50% of the records from the "Customers" table:

Example

SELECT TOP 50 PERCENT * FROM Customers;

SQL LIKE Operator

The LIKE operator is used in a WHERE clause to search for a the specified pattern in a column.

The SQL LIKE Operator

The LIKE operator is used to search for a specified pattern in a column.

SQL LIKE Syntax

SELECT column_name(s)
FROM table_name
WHERE column_name LIKE pattern;

Demo Database

In this tutorial, we will use the well-known Northwind sample database.

Below is a selection from the "Customers" table:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin	12209	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constitución 2222	México D.F.	05021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	05023	Mexico
4	Around the Horn	Thomas Hardy	120 Hanover Sq.	London	WA1 1DP	UK
5	Berglunds snabbköp	Christina Berglund	Berguvsvägen 8	Luleå	S-958 22	Sweden

SQL LIKE Operator Examples

The following SQL statement selects all customers with a City starting with the letter "s":

Example

SELECT * FROM Customers
WHERE City LIKE 's%';

Tip: The "%" sign is used to define wildcards (missing letters) both before and after the pattern. You will learn more about wildcards in the next chapter.

The following SQL statement selects all customers with a City ending with the letter "s":

Example

SELECT * FROM Customers
WHERE City LIKE '%s';

The following SQL statement selects all customers with a Country containing the pattern "land":

Example

SELECT * FROM Customers
WHERE Country LIKE '%land%';

Using the NOT keyword allows you to select records that do NOT match the pattern.

The following SQL statement selects all customers with a Country NOT containing the pattern "land":

Example

SELECT * FROM Customers
WHERE Country NOT LIKE '%land%';

SQL Wildcards

A wildcard character can be used to substitute for any other character(s) in a string.

SQL Wildcard Characters

In SQL, wildcard characters are used with the SQL LIKE operator.

SQL wildcards are used to search for data within a table.

With SQL, the wildcards are:

Wildcard	Description
%	A substitute for zero or more characters
_	A substitute for a single character
[charlist]	Sets and ranges of characters to match
[^charlist] or [!charlist]	Matches only a character NOT specified within the brackets

Demo Database

In this tutorial, we will use the well-known Northwind sample database.

Below is a selection from the "Customers" table:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin	12209	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constitución 2222	México D.F.	05021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	05023	Mexico
4	Around the Horn	Thomas Hardy	120 Hanover Sq.	London	WA1 1DP	UK
5	Berglunds snabbköp	Christina Berglund	Berguvsvägen 8	Luleå	S-958 22	Sweden

Using the SQL % Wildcard

The following SQL statement selects all customers with a City starting with "ber":

Example

SELECT * FROM Customers
WHERE City LIKE 'ber%';

The following SQL statement selects all customers with a City containing the pattern "es":

Example

SELECT * FROM Customers
WHERE City LIKE '%es%';

Using the SQL _ Wildcard

The following SQL statement selects all customers with a City starting with any character, followed by "merlin":

Example

SELECT * FROM Customers
WHERE City LIKE '_erlin';

The following SQL statement selects all customers with a City starting with "L", followed by any character, followed by "n", followed by any character, followed by "on":

Example

SELECT * FROM Customers
WHERE City LIKE 'L_n_on';

Using the SQL [charlist] Wildcard

The following SQL statement selects all customers with a City starting with "b", "s", or "p":

Example

SELECT * FROM Customers
WHERE City LIKE '[bsp]%';

The following SQL statement selects all customers with a City starting with "a", "b", or "c":

Example

SELECT * FROM Customers
WHERE City LIKE '[a-c]%';

The following SQL statement selects all customers with a City NOT starting with "b", "s", or "p":

Example

SELECT * FROM Customers
WHERE City LIKE '[!bsp]%';

SQL IN Operator

The IN Operator

The IN operator allows you to specify multiple values in a WHERE clause.

SQL IN Syntax

SELECT column_name(s)
FROM table_name
WHERE column_name IN (value1,value2,...);

Demo Database

In this tutorial, we will use the well-known Northwind sample database.

Below is a selection from the "Customers" table:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin	12209	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constitución 2222	México D.F.	05021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	05023	Mexico
4	Around the Horn	Thomas Hardy	120 Hanover Sq.	London	WA1 1DP	UK
5	Berglunds snabbköp	Christina Berglund	Berguvsvägen 8	Luleå	S-958 22	Sweden

IN Operator Example

The following SQL statement selects all customers with a City of "Paris" or "London":

Example

SELECT * FROM Customers
WHERE City IN ('Paris', 'London');

SQL BETWEEN Operator

The BETWEEN operator is used to select values within a range.

The SQL BETWEEN Operator

The BETWEEN operator selects values within a range. The values can be numbers, text, or dates.

SQL BETWEEN Syntax

SELECT column_name(s)
FROM table_name
WHERE column_name BETWEEN value1 AND value2;

Demo Database

In this tutorial, we will use the well-known Northwind sample database.

Below is a selection from the "Products" table:

ProductID	ProductName	SupplierID	CategoryID	Unit	Price
1	Chais	1	1	10 boxes x 20 bags	18
2	Chang	1	1	24 - 12 oz bottles	19
3	Aniseed Syrup	1	2	12 - 550 ml bottles	10
4	Chef Anton's Cajun Seasoning	1	2	48 - 6 oz jars	22
5	Chef Anton's Gumbo Mix	1	2	36 boxes	21.35

BETWEEN Operator Example

The following SQL statement selects all products with a price BETWEEN 10 and 20:

Example

SELECT * FROM Products
WHERE Price BETWEEN 10 AND 20;

NOT BETWEEN Operator Example

To display the products outside the range of the previous example, use NOT BETWEEN:

Example

SELECT * FROM Products
WHERE Price NOT BETWEEN 10 AND 20;

BETWEEN Operator with IN Example

The following SQL statement selects all products with a price BETWEEN 10 and 20, but products with a CategoryID of 1,2, or 3 should not be displayed:

Example

SELECT * FROM Products
WHERE (Price BETWEEN 10 AND 20)
AND NOT CategoryID IN (1,2,3);

BETWEEN Operator with Text Value Example

The following SQL statement selects all products with a ProductName beginning with any of the letters BETWEEN 'C' and 'M':

Example

SELECT * FROM Products
WHERE ProductName BETWEEN 'C' AND 'M';

NOT BETWEEN Operator with Text Value Example

The following SQL statement selects all products with a ProductName beginning with any of the letters NOT BETWEEN 'C' and 'M':

Example

SELECT * FROM Products
WHERE ProductName NOT BETWEEN 'C' AND 'M';

Sample Table

Below is a selection from the "Orders" table:

OrderID	CustomerID	EmployeeID	OrderDate	shipped
10248	90	5	7/4/1996	3
10249	81	6	7/5/1996	1
10250	34	4	7/8/1996	2
10251	84	3	7/9/1996	1
10252	76	4	7/10/1996	2

BETWEEN Operator with Date Value Example

The following SQL statement selects all orders with an OrderDate BETWEEN '04-July-1996' and '09-July-1996':

Example

SELECT * FROM Orders
WHERE OrderDate BETWEEN #07/04/1996# AND #07/09/1996#;

Notice that the BETWEEN operator can produce different results in different databases!
In some databases, BETWEEN selects fields that are between and excluding the test values.
In other databases, BETWEEN selects fields that are between and including the test values.
And in other databases, BETWEEN selects fields between the test values, including the first test value and excluding the last test value.

Therefore: Check how your database treats the BETWEEN operator!

SQL Aliases

SQL aliases are used to temporarily rename a table or a column heading.

SQL Aliases

SQL aliases are used to give a database table or a column in a table a temporarily name.

Basically, aliases are created to make column names more readable.

SQL Alias Syntax for Columns

SELECT column_name AS alias_name
FROM table_name;

SQL Alias Syntax for Tables

SELECT column_name(s)
FROM table_name AS alias_name;

Demo Database

In this tutorial we will use the well-known Northwind sample database.

Below is a selection from the "Customers" table:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin	12209	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constitución 2222	México D.F.	05021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	05023	Mexico

And a selection from the "Orders" table:

OrderID	CustomerID	EmployeeID	OrderDate	ShipperID
10643	1	6	1997-08-25	1
10644	88	3	1997-08-25	2
10645	34	4	1997-08-26	1

Alias Example for Table Columns

The following SQL statement specifies two aliases, one for the CustomerName column and one for the ContactName column. Tip: It requires double quotation marks or square brackets if the column name contains spaces:

Example

SELECT CustomerName AS Customer, ContactName AS [Contact Person]
FROM Customers;

In the following SQL statement, we combine four columns (Address, City, PostalCode, and Country) and create an alias named "Address":

Example

SELECT CustomerName, Address+', '+City+', '+PostalCode+', '+Country AS Address
FROM Customers;

Alias Example for Tables

The following SQL statement selects all the orders from the customer "Alfreds Futterkiste". We use the "Customers" and "Orders" tables, and give them the table aliases of "c" and "o" respectively (Here we have used aliases to make the SQL shorter):

Example

SELECT o.OrderID, o.OrderDate, c.CustomerName
FROM Customers AS c, Orders AS o
WHERE c.CustomerName='Alfreds Futterkiste';

The same SQL statement without aliases:

Example

SELECT Orders.OrderID, Orders.OrderDate, Customers.CustomerName
FROM Customers, Orders
WHERE Customers.CustomerName='Alfreds Futterkiste';

Aliases can be useful when:

There are more than one table involved in a query
Functions are used in the query
Column names are big or not very readable
Two or more columns are combined together

SQL Joins

SQL joins are used to combine rows from two or more tables.

SQL JOIN

An SQL JOIN clause is used to combine rows from two or more tables, based on a common field between them.

The most common type of join is: SQL INNER JOIN (simple join). An SQL INNER JOIN return all rows from multiple tables where the join condition is met.

Let's look at a selection from the "Orders" table:

OrderID	CustomerID	OrderDate
10308	2	1996-09-18
10309	37	1996-09-19
10310	77	1996-09-20

Then, have a look at a selection from the "Customers" table:

CustomerID	CustomerName	ContactName	Country
1	Alfreds Futterkiste	Maria Anders	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mexico

Notice that the "CustomerID" column in the "Orders" table refers to the customer in the "Customers" table. The relationship between the two tables above is the "CustomerID" column.

Then, if we run the following SQL statement (that contains an INNER JOIN):

Example

SELECT Orders.OrderID, Customers.CustomerName, Orders.OrderDate
FROM Orders
INNER JOIN Customers
ON Orders.CustomerID=Customers.CustomerID;

it will produce something like this:

OrderID	CustomerName	OrderDate
10308	Ana Trujillo Emparedados y helados	9/18/1996
10365	Antonio Moreno Taquería	11/27/1996
10383	Around the Horn	12/16/1996
10355	Around the Horn	11/15/1996
10278	Berglunds snabbköp	8/12/1996

Different SQL JOINs

Before we continue with examples, we will list the types the different SQL JOINs you can use:

INNER JOIN: Returns all rows when there is at least one match in BOTH tables
LEFT JOIN: Return all rows from the left table, and the matched rows from the right table
RIGHT JOIN: Return all rows from the right table, and the matched rows from the left table
FULL JOIN: Return all rows when there is a match in ONE of the tables

SQL INNER JOIN Keyword

The INNER JOIN keyword selects all rows from both tables as long as there is a match between the columns in both tables.

SQL INNER JOIN Syntax

SELECT column_name(s)
FROM table1
INNER JOIN table2
ON table1.column_name=table2.column_name;

or:

SELECT column_name(s)
FROM table1
JOIN table2
ON table1.column_name=table2.column_name;

PS! INNER JOIN is the same as JOIN.

Demo Database

In this tutorial we will use the well-known Northwind sample database.

Below is a selection from the "Customers" table:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin	12209	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constitución 2222	México D.F.	05021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	05023	Mexico

And a selection from the "Orders" table:

OrderID	CustomerID	EmployeeID	OrderDate	ShipperID
10308	2	7	1996-09-18	3
10309	37	3	1996-09-19	1
10310	77	8	1996-09-20	2

SQL INNER JOIN Example

The following SQL statement will return all customers with orders:

Example

SELECT Customers.CustomerName, Orders.OrderID
FROM Customers
INNER JOIN Orders
ON Customers.CustomerID=Orders.CustomerID
ORDER BY Customers.CustomerName;

Note: The INNER JOIN keyword selects all rows from both tables as long as there is a match between the columns. If there are rows in the "Customers" table that do not have matches in "Orders", these customers will NOT be listed.

SQL LEFT JOIN Keyword

The LEFT JOIN keyword returns all rows from the left table (table1), with the matching rows in the right table (table2). The result is NULL on the right side when there is no match.

SQL LEFT JOIN Syntax

SELECT column_name(s)
FROM table1
LEFT JOIN table2
ON table1.column_name=table2.column_name;

or:

SELECT column_name(s)
FROM table1
LEFT OUTER JOIN table2
ON table1.column_name=table2.column_name;

PS! In some databases LEFT JOIN is called LEFT OUTER JOIN.

Demo Database

In this tutorial we will use the well-known Northwind sample database.

Below is a selection from the "Customers" table:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin	12209	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constitución 2222	México D.F.	05021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	05023	Mexico

And a selection from the "Orders" table:

OrderID	CustomerID	EmployeeID	OrderDate	ShipperID
10308	2	7	1996-09-18	3
10309	37	3	1996-09-19	1
10310	77	8	1996-09-20	2

SQL LEFT JOIN Example

The following SQL statement will return all customers, and any orders they might have:

Example

SELECT Customers.CustomerName, Orders.OrderID
FROM Customers
LEFT JOIN Orders
ON Customers.CustomerID=Orders.CustomerID
ORDER BY Customers.CustomerName;

Note: The LEFT JOIN keyword returns all the rows from the left table (Customers), even if there are no matches in the right table (Orders).

SQL FULL OUTER JOIN Keyword

The FULL OUTER JOIN keyword returns all rows from the left table (table1) and from the right table (table2).

The FULL OUTER JOIN keyword combines the result of both LEFT and RIGHT joins.

SQL FULL OUTER JOIN Syntax

SELECT column_name(s)
FROM table1
FULL OUTER JOIN table2
ON table1.column_name=table2.column_name;

Demo Database

In this tutorial we will use the well-known Northwind sample database.

Below is a selection from the "Customers" table:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin	12209	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constitución 2222	México D.F.	05021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	05023	Mexico

And a selection from the "Orders" table:

OrderID	CustomerID	EmployeeID	OrderDate	ShipperID
10308	2	7	1996-09-18	3
10309	37	3	1996-09-19	1
10310	77	8	1996-09-20	2

SQL FULL OUTER JOIN Example

The following SQL statement selects all customers and all orders:

SELECT Customers.CustomerName, Orders.OrderID
FROM Customers
FULL OUTER JOIN Orders
ON Customers.CustomerID=Orders.CustomerID
ORDER BY Customers.CustomerName;

A selection from the result set may look like this:

CustomerName	OrderID
Alfreds Futterkiste
Ana Trujillo Emparedados y helados	10308
Antonio Moreno Taquería	10365
	10382
	10351

Note: The FULL OUTER JOIN keyword returns all the rows from the left table (Customers), and all the rows from the right table (Orders). If there are rows in "Customers" that do not have matches in "Orders", or if there are rows in "Orders" that do not have matches in "Customers", those rows will be listed as well.

SQL UNION Operator

The SQL UNION operator combines two or more SELECT statements.

The SQL UNION Operator

The UNION operator is used to combine the result-set of two or more SELECT statements.

Notice that each SELECT statement within the UNION must have the same number of columns. The columns must also have similar data types. Also, the columns in each SELECT statement must be in the same order.

SQL UNION Syntax

SELECT column_name(s) FROM table_name1
UNION
SELECT column_name(s) FROM table_name2;

Note: The UNION operator selects only distinct values by default. To allow duplicate values, use UNION ALL.

SQL UNION ALL Syntax

SELECT column_name(s) FROM table_name1
UNION ALL
SELECT column_name(s) FROM table_name2;

PS: The column names in the result-set of a UNION are always equal to the column names in the first SELECT statement in the UNION.

Demo Database

In this tutorial, we will use the well-known Northwind sample database.

Below is a selection from the "Customers" table:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin	12209	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constitución 2222	México D.F.	05021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	05023	Mexico

And a selection from the "Suppliers" table:

SupplierID	SupplierName	ContactName	Address	City	PostalCode	Country
1	Exotic Liquid	Charlotte Cooper	49 Gilbert St.	Londona	EC1 4SD	UK
2	New Orleans Cajun Delights	Shelley Burke	P.O. Box 78934	New Orleans	70117	USA
3	Grandma Kelly's Homestead	Regina Murphy	707 Oxford Rd.	Ann Arbor	48104	USA

SQL UNION Example

The following SQL statement selects all the different cities from the "Customers" and "Suppliers" tables:

Example

SELECT City FROM Customers
UNION
SELECT City FROM Suppliers;

Note: This command cannot be used to list all cities from the two tables. In the example above several customers and suppliers share the same city, but each city will only be listed once. The UNION command selects only distinct values.

SQL UNION ALL Example

The following SQL statement selects all the cities from the "Customers" and "Suppliers" tables:

Example

SELECT City FROM Customers
UNION ALL
SELECT City FROM Suppliers
ORDER BY City;

SQL CREATE DATABASE Statement

The CREATE DATABASE Statement

The CREATE DATABASE statement is used to create a database.

SQL CREATE DATABASE Syntax

CREATE DATABASE database_name

CREATE DATABASE Example

Now we want to create a database called "my_db".

We use the following CREATE DATABASE statement:

CREATE DATABASE my_db

Database tables can be added with the CREATE TABLE statement.

SQL CREATE TABLE Statement

The CREATE TABLE Statement

The CREATE TABLE statement is used to create a table in a database.

SQL CREATE TABLE Syntax

CREATE TABLE table_name
(
column_name1 data_type,
column_name2 data_type,
column_name3 data_type,
....
)

The data type specifies what type of data the column can hold. For a complete reference of all the data types available in MS Access, MySQL, and SQL Server

CREATE TABLE Example

Now we want to create a table called "Persons" that contains five columns: P_Id, LastName, FirstName, Address, and City.

We use the following CREATE TABLE statement:

CREATE TABLE Persons
(
P_Id int,
LastName varchar(255),
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)

The P_Id column is of type int and will hold a number. The LastName, FirstName, Address, and City columns are of type varchar with a maximum length of 255 characters.

SQL Constraints

Constraints are used to limit the type of data that can go into a table.

Constraints can be specified when a the table is created (with the CREATE TABLE statement) or after the table is created (with the ALTER TABLE statement).

We will focus on the following constraints:

NOT NULL
UNIQUE
PRIMARY KEY
FOREIGN KEY
CHECK
DEFAULT

SQL NOT NULL Constraint

By default, a table column can hold NULL values.

SQL NOT NULL Constraint

The NOT NULL constraint enforces a column to NOT accept NULL values.

The NOT NULL constraint enforces a field to always contain a value. This means that you cannot insert a new record, or update a record without adding a value to this field.

The following SQL enforces the "P_Id" column and the "LastName" column to not accept NULL values:

CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)

SQL UNIQUE Constraint

The UNIQUE constraint uniquely identifies each record in a database table.

The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns.

A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.

Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.

SQL UNIQUE Constraint on CREATE TABLE

The following SQL creates a UNIQUE constraint on the "P_Id" column when the "Persons" table is created:

MySQL:

CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
UNIQUE (P_Id)
)

SQL Server / Oracle / MS Access:

CREATE TABLE Persons
(
P_Id int NOT NULL UNIQUE,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)

To allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)
)

SQL UNIQUE Constraint on ALTER TABLE

To create a UNIQUE constraint on the "P_Id" column when the table is already created, use the following SQL:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ADD UNIQUE (P_Id)

To allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ADD CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)

To DROP a UNIQUE Constraint

To drop a UNIQUE constraint, use the following SQL:

MySQL:

ALTER TABLE Persons
DROP INDEX uc_PersonID

SQL Server / Oracle / MS Access:

ALTER TABLE Persons
DROP CONSTRAINT uc_PersonID

SQL PRIMARY KEY Constraint

The PRIMARY KEY constraint uniquely identifies each record in a database table.

Primary keys must contain unique values.

A primary key column cannot contain NULL values.

Each table should have a primary key and each table can have only ONE primary key.

SQL PRIMARY KEY Constraint on CREATE TABLE

The following SQL creates a PRIMARY KEY on the "P_Id" column when the "Persons" table is created:

MySQL:

CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
PRIMARY KEY (P_Id)
)

SQL Server / Oracle / MS Access:

CREATE TABLE Persons
(
P_Id int NOT NULL PRIMARY KEY,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)

To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
)

Note: In the example above there is only ONE PRIMARY KEY (pk_PersonID). However, the value of the pk_PersonID is made up of two columns (P_Id and LastName).

SQL PRIMARY KEY Constraint on ALTER TABLE

To create a PRIMARY KEY constraint on the "P_Id" column when the table is already created, use the following SQL:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ADD PRIMARY KEY (P_Id)

To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ADD CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)

Note: If you use the ALTER TABLE statement to add a primary key, the primary key column(s) must already have been declared to not contain NULL values (when the table was first created).

To DROP a PRIMARY KEY Constraint

To drop a PRIMARY KEY constraint, use the following SQL:

MySQL:

ALTER TABLE Persons
DROP PRIMARY KEY

SQL Server / Oracle / MS Access:

ALTER TABLE Persons
DROP CONSTRAINT pk_PersonID

SQL FOREIGN KEY Constraint

A FOREIGN KEY in one table points to a PRIMARY KEY in another table.

Let's illustrate the foreign key with an example. Look at the following two tables:

The "Persons" table:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgen 23	Sandnes
3	Pettersen	Kari	Storage 20	Stavanger

The "Orders" table:

O_Id	OrderNo	P_Id
1	77895	3
2	44678	3
3	22456	2
4	24562	1

Note that the "P_Id" column in the "Orders" table points to the "P_Id" column in the "Persons" table.

The "P_Id" column in the The "Persons" table is the PRIMARY KEY in the "Persons" table.

The "P_Id" column in the The "Orders" table is a FOREIGN KEY in the "Orders" table.

The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables.

The FOREIGN KEY constraint also prevents invalid data from being inserted into the foreign key column, because it has to be one of the values contained in the table it points to.

SQL FOREIGN KEY Constraint on CREATE TABLE

The following SQL creates a FOREIGN KEY on the "P_Id" column when the "Orders" table is created:

MySQL:

CREATE TABLE Orders
(
O_Id int NOT NULL,
OrderNo int NOT NULL,
P_Id int,
PRIMARY KEY (O_Id),
FOREIGN KEY (P_Id) REFERENCES Persons(P_Id)
)

SQL Server / Oracle / MS Access:

CREATE TABLE Orders
(
O_Id int NOT NULL PRIMARY KEY,
OrderNo int NOT NULL,
P_Id int FOREIGN KEY REFERENCES Persons(P_Id)
)

To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Orders
(
O_Id int NOT NULL,
OrderNo int NOT NULL,
P_Id int,
PRIMARY KEY (O_Id),
CONSTRAINT fk_PerOrders FOREIGN KEY (P_Id)
REFERENCES Persons(P_Id)
)

SQL FOREIGN KEY Constraint on ALTER TABLE

To create a FOREIGN KEY constraint on the "P_Id" column when the "Orders" table is already created, use the following SQL:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Orders
ADD FOREIGN KEY (P_Id)
REFERENCES Persons(P_Id)

To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Orders
ADD CONSTRAINT fk_PerOrders
FOREIGN KEY (P_Id)
REFERENCES Persons(P_Id)

To DROP a FOREIGN KEY Constraint

To drop a FOREIGN KEY constraint, use the following SQL:

MySQL:

ALTER TABLE Orders
DROP FOREIGN KEY fk_PerOrders

SQL Server / Oracle / MS Access:

ALTER TABLE Orders
DROP CONSTRAINT fk_PerOrders

SQL CHECK Constraint

The CHECK constraint is used to limit the value range that can be placed in a column.

If you define a CHECK constraint on a single column it allows only certain values for this column.

If you define a CHECK constraint on a table it can limit the values in certain columns based on values in other columns in the row.

SQL CHECK Constraint on CREATE TABLE

The following SQL creates a CHECK constraint on the "P_Id" column when the "Persons" table is created. The CHECK constraint specifies that the column "P_Id" must only include integers greater than 0.

MySQL:

CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CHECK (P_Id>0)
)

SQL Server / Oracle / MS Access:

CREATE TABLE Persons
(
P_Id int NOT NULL CHECK (P_Id>0),
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)

To allow naming of a CHECK constraint, and for defining a CHECK constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT chk_Person CHECK (P_Id>0 AND City='Sandnes')
)

SQL CHECK Constraint on ALTER TABLE

To create a CHECK constraint on the "P_Id" column when the table is already created, use the following SQL:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ADD CHECK (P_Id>0)

To allow naming of a CHECK constraint, and for defining a CHECK constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ADD CONSTRAINT chk_Person CHECK (P_Id>0 AND City='Sandnes')

To DROP a CHECK Constraint

To drop a CHECK constraint, use the following SQL:

SQL Server / Oracle / MS Access:

ALTER TABLE Persons
DROP CONSTRAINT chk_Person

MySQL:

ALTER TABLE Persons
DROP CHECK chk_Person

SQL DEFAULT Constraint

The DEFAULT constraint is used to insert a default value into a column.

The default value will be added to all new records, if no other value is specified.

SQL DEFAULT Constraint on CREATE TABLE

The following SQL creates a DEFAULT constraint on the "City" column when the "Persons" table is created:

My SQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255) DEFAULT 'Sandnes'
)

The DEFAULT constraint can also be used to insert system values, by using functions like GETDATE():

CREATE TABLE Orders
(
O_Id int NOT NULL,
OrderNo int NOT NULL,
P_Id int,
OrderDate date DEFAULT GETDATE()
)

SQL DEFAULT Constraint on ALTER TABLE

To create a DEFAULT constraint on the "City" column when the table is already created, use the following SQL:

MySQL:

ALTER TABLE Persons
ALTER City SET DEFAULT 'SANDNES'

SQL Server / MS Access:

ALTER TABLE Persons
ALTER COLUMN City SET DEFAULT 'SANDNES'

Oracle:

ALTER TABLE Persons
MODIFY City DEFAULT 'SANDNES'

To DROP a DEFAULT Constraint

To drop a DEFAULT constraint, use the following SQL:

MySQL:

ALTER TABLE Persons
ALTER City DROP DEFAULT

SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ALTER COLUMN City DROP DEFAULT

SQL ALTER TABLE Statement

The ALTER TABLE Statement

The ALTER TABLE statement is used to add, delete, or modify columns in an existing table.

SQL ALTER TABLE Syntax

To add a column in a table, use the following syntax:

ALTER TABLE table_name
ADD column_name datatype

To delete a column in a table, use the following syntax (notice that some database systems don't allow deleting a column):

ALTER TABLE table_name
DROP COLUMN column_name

To change the data type of a column in a table, use the following syntax:

SQL Server / MS Access:

ALTER TABLE table_name
ALTER COLUMN column_name datatype

My SQL / Oracle:

ALTER TABLE table_name
MODIFY column_name datatype

SQL ALTER TABLE Example

Look at the "Persons" table:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgvn 23	Sandnes
3	Pettersen	Kari	Storgt 20	Stavanger

Now we want to add a column named "DateOfBirth" in the "Persons" table.

We use the following SQL statement:

ALTER TABLE Persons
ADD DateOfBirth date

Notice that the new column, "DateOfBirth", is of type date and is going to hold a date. The data type specifies what type of data the column can hold. For a complete reference of all the data types available in MS Access, MySQL and SQL ServerThe "Persons" table will now like this:

P_Id	LastName	FirstName	Address	City	DateOfBirth
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgen 23	Sandnes
3	Pettersen	Kari	Storage 20	Stavanger

Change Data Type Example

Now we want to change the data type of the column named "DateOfBirth" in the "Persons" table.

We use the following SQL statement:

ALTER TABLE Persons
ALTER COLUMN DateOfBirth year

Notice that the "DateOfBirth" column is now of type year and is going to hold a year in a two-digit or four-digit format.

DROP COLUMN Example

Next, we want to delete the column named "DateOfBirth" in the "Persons" table.

We use the following SQL statement:

ALTER TABLE Persons
DROP COLUMN DateOfBirth

The "Persons" table will now like this:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola	Timoteivn 10	Sandnes
2	Svendson	Tove	Borgen 23	Sandnes
3	Pettersen	Kari	Story 20	Stavanger

SQL AUTO INCREMENT Field

Auto-increment allows a unique number to be generated when a new record is inserted into a table.

AUTO INCREMENT a Field

Very often we would like the value of the primary key field to be created automatically every time a new record is inserted.

We would like to create an auto-increment field in a table.

Syntax for MySQL

The following SQL statement defines the "P_Id" column to be an auto-increment primary key field in the "Persons" table:

CREATE TABLE Persons
(
P_Id int NOT NULL AUTO_INCREMENT,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
PRIMARY KEY (P_Id)
)

MySQL uses the AUTO_INCREMENT keyword to perform an auto-increment feature.

By default, the starting value for AUTO_INCREMENT is 1, and it will increment by 1 for each new record.

To let the AUTO_INCREMENT sequence start with another value, use the following SQL statement:

ALTER TABLE Persons AUTO_INCREMENT=100

To insert a new record into the "Persons" table, we will not have to specify a value for the "P_Id" column (a unique value will be added automatically):

INSERT INTO Persons (FirstName,LastName)
VALUES ('Lars','Monsen')

The SQL statement above would insert a new record into the "Persons" table. The "P_Id" column would be assigned a unique value. The "FirstName" column would be set to "Lars" and the "LastName" column would be set to "Monsen".

Syntax for SQL Server

The following SQL statement defines the "P_Id" column to be an auto-increment primary key field in the "Persons" table:

CREATE TABLE Persons
(
P_Id int PRIMARY KEY IDENTITY,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)

The MS SQL Server uses the IDENTITY keyword to perform an auto-increment feature.

By default, the starting value for IDENTITY is 1, and it will increment by 1 for each new record.

To specify that the "P_Id" the column should start at value 10 and increment by 5, change the identity to IDENTITY(10,5).

To insert a new record into the "Persons" table, we will not have to specify a value for the "P_Id" column (a unique value will be added automatically):

INSERT INTO Persons (FirstName,LastName)
VALUES ('Lars','Monsen')

Syntax for Access

The following SQL statement defines the "P_Id" column to be an auto-increment primary key field in the "Persons" table:

CREATE TABLE Persons
(
P_Id PRIMARY KEY AUTOINCREMENT,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)

MS Access uses the AUTOINCREMENT keyword to perform an auto-increment feature.

By default, the starting value for AUTOINCREMENT is 1, and it will increment by 1 for each new record.

To specify that the "P_Id" the column should start at value 10 and increment by 5, change the autoincrement to AUTOINCREMENT(10,5).

To insert a new record into the "Persons" table, we will not have to specify a value for the "P_Id" column (a unique value will be added automatically):

INSERT INTO Persons (FirstName,LastName)
VALUES ('Lars','Monsen')

Syntax for Oracle

In Oracle, the code is a little bit more tricky.

You will have to create an auto-increment field with the sequence object (this object generates a number sequence).

Use the following CREATE SEQUENCE syntax:

CREATE SEQUENCE seq_person
MIN VALUE 1
START WITH 1
INCREMENT BY 1
CACHE 10

The code above creates a sequence of an object called seq_person, that starts with 1 and will increment by 1. It will also cache up to 10 values for performance. The cache option specifies how many sequence values will be stored in memory for faster access.

To insert a new record into the "Persons" table, we will have to use the nextval function (this function retrieves the next value from seq_person sequence):

INSERT INTO Persons (P_Id,FirstName,LastName)
VALUES (seq_person.nextval,'Lars','Monsen')

The SQL statement above would insert a new record into the "Persons" table. The "P_Id" column would be assigned the next number from the seq_person sequence. The "FirstName" column would be set to "Lars" and the "LastName" column would be set to "Monsen".

SQL Views

A view is a virtual table.

This chapter shows how to create, update, and delete a view.

SQL CREATE VIEW Statement

In SQL, a view is a virtual table based on the result-set of an SQL statement.

A view contains rows and columns, just like a real table. The fields in a view are fields from one or more real tables in the database.

You can add SQL functions, WHERE, and JOIN statements to a view and present the data as if the data were coming from one single table.

SQL CREATE VIEW Syntax

CREATE VIEW view_name AS
SELECT column_name(s)
FROM table_name
WHERE condition

Note: A view always shows up-to-date data! The database engine recreates the data, using the view's SQL statement, every time a user queries a view.

SQL CREATE VIEW Examples

If you have the Northwind database you can see that it has several views installed by default.

The view "Current Product List" lists all active products (products that are not discontinued) from the "Products" table. The view is created with the following SQL:

CREATE VIEW [Current Product List] AS
SELECT ProductID,ProductName
FROM Products
WHERE Discontinued=No

We can query the view above as follows:

SELECT * FROM [Current Product List]

Another view in the Northwind sample database selects every product in the "Products" table with a unit price higher than the average unit price:

CREATE VIEW [Products Above Average Price] AS
SELECT ProductName,UnitPrice
FROM Products
WHERE UnitPrice>(SELECT AVG(UnitPrice) FROM Products)

We can query the view above as follows:

SELECT * FROM [Products Above Average Price]

Another view in the Northwind database calculates the total sale for each category in 1997. Note that this view selects its data from another view called "Product Sales for 1997":

CREATE VIEW [Category Sales For 1997] AS
SELECT DISTINCT CategoryName,Sum(ProductSales) AS CategorySales
FROM [Product Sales for 1997]
GROUP BY CategoryName

We can query the view above as follows:

SELECT * FROM [Category Sales For 1997]

We can also add a condition to the query. Now we want to see the total sale only for the category "Beverages":

SELECT * FROM [Category Sales For 1997]
WHERE CategoryName='Beverages'

SQL Updating a View

You can update a view by using the following syntax:

SQL CREATE OR REPLACE VIEW Syntax

CREATE OR REPLACE VIEW view_name AS
SELECT column_name(s)
FROM table_name
WHERE condition

Now we want to add the "Category" column to the "Current Product List" view. We will update the view with the following SQL:

CREATE VIEW [Current Product List] AS
SELECT ProductID,ProductName,Category
FROM Products
WHERE Discontinued=No

SQL Dropping a View

You can delete a view with the DROP VIEW command.

SQL DROP VIEW Syntax

DROP VIEW view_name

SQL NULL Values

NULL values represent missing unknown data.

By default, a table column can hold NULL values.

This chapter will explain the IS NULL and IS NOT NULL operators.

SQL NULL Values

If a column in a table is optional, we can insert a new record or update an existing record without adding a value to this column. This means that the field will be saved with a NULL value.

NULL values are treated differently from other values.

NULL is used as a placeholder for unknown or inapplicable values.

Note: It is not possible to compare NULL and 0; they are not equivalent.

SQL Working with NULL Values

Look at the following "Persons" table:

P_Id	LastName	FirstName	Address	City
1	Hansen	Ola		Sandnes
2	Svendson	Tove	Borgen 23	Sandnes
3	Pettersen	Kari		Stavanger

Suppose that the "Address" column in the "Persons" table is optional. This means that if we insert a record with no value for the "Address" column, the The "Address" column will be saved with a NULL value.

How can we test for NULL values?

It is not possible to test for NULL values with comparison operators, such as =, <, or <>.

We will have to use the IS NULL and IS NOT NULL operators instead.

SQL IS NULL

How do we select only the records with NULL values in the "Address" column?

We will have to use the IS NULL operator:

SELECT LastName,FirstName,Address FROM Persons
WHERE Address IS NULL

The result-set will look like this:

LastName	FirstName	Address
Hansen	Ola
Pettersen	Kari

Tip: Always use IS NULL to look for NULL values.

SQL IS NOT NULL

How do we select only the records with no NULL values in the "Address" column?

We will have to use the IS NOT NULL operator:

SELECT LastName,FirstName,Address FROM Persons
WHERE Address IS NOT NULL

The result-set will look like this:

LastName	FirstName	Address
Svendson	Tove	Borgen 23

Iwe will look at the ISNULL(), NVL(), IFNULL() and COALESCE() functions.

SQL NULL Functions

SQL ISNULL(), NVL(), IFNULL() and COALESCE() Functions

Look at the following "Products" table:

P_Id	ProductName	UnitPrice	UnitsInStock	UnitsOnOrder
1	Jarlsberg	10.45	16	15
2	Mascarpone	32.56	23
3	Gorgonzola	15.67	9	20

Suppose that the "UnitsOnOrder" column is optional, and may contain NULL values.

We have the following SELECT statement:

SELECT ProductName,UnitPrice*(UnitsInStock+UnitsOnOrder)
FROM Products

In the example above, if any of the "UnitsOnOrder" values are NULL, the result is NULL.

Microsoft's ISNULL() function is used to specify how we want to treat NULL values.

The NVL(), IFNULL(), and COALESCE() functions can also be used to achieve the same result.

In this case we want NULL values to be zero.

Below, if "UnitsOnOrder" is NULL it will not harm the calculation, because ISNULL() returns a zero if the value is NULL:

SQL Server / MS Access

SELECT ProductName,UnitPrice*(UnitsInStock+ISNULL(UnitsOnOrder,0))
FROM Products

Oracle

Oracle does not have an ISNULL() function. However, we can use the NVL() function to achieve the same result:

SELECT ProductName,UnitPrice*(UnitsInStock+NVL(UnitsOnOrder,0))
FROM Products

MySQL

MySQL does have an ISNULL() function. However, it works a little bit different from Microsoft's ISNULL() function.

In MySQL we can use the IFNULL() function, like this:

SELECT ProductName,UnitPrice*(UnitsInStock+IFNULL(UnitsOnOrder,0))
FROM Products

or we can use the COALESCE() function, like this:

SELECT ProductName,UnitPrice*(UnitsInStock+COALESCE(UnitsOnOrder,0))
FROM Products

SQL General Data Types

A data type defines what kind of value a column can contain.

SQL General Data Types

Each column in a database table is required to have a name and a data type.

SQL developers have to decide what types of data will be stored inside each and every table column when creating a SQL table. The data type is a label and a guideline for SQL to understand what type of data is expected inside of each column, and it also identifies how SQL will interact with the stored data.

The following table lists the general data types in SQL:

Data type	Description
CHARACTER(n)	Character string. Fixed-length n
VARCHAR(n) or CHARACTER VARYING(n)	Character string. Variable length. Maximum length n
BINARY(n)	Binary string. Fixed-length n
BOOLEAN	Stores TRUE or FALSE values
VARBINARY(n) or BINARY VARYING(n)	Binary string. Variable length. Maximum length n
INTEGER(p)	Integer numerical (no decimal). Precision p
SMALLINT	Integer numerical (no decimal). Precision 5
INTEGER	Integer numerical (no decimal). Precision 10
BIGINT	Integer numerical (no decimal). Precision 19
DECIMAL(p,s)	Exact numerical, precision p, scale s. Example: decimal(5,2) is a number that has 3 digits before the decimal and 2 digits after the decimal
NUMERIC(p,s)	Exact numerical, precision p, scale s. (Same as DECIMAL)
FLOAT(p)	Approximate numerical, mantissa precision p. A floating number in base 10 exponential notation. The size the argument for this type consists of a single number specifying the minimum precision
REAL	Approximate numerical, mantissa precision 7
FLOAT	Approximate numerical, mantissa precision 16
DOUBLE PRECISION	Approximate numerical, mantissa precision 16
DATE	Stores year, month, and day values
TIME	Stores hour, minute, and second values
TIMESTAMP	Stores year, month, day, hour, minute, and second values
INTERVAL	Composed of a number of integers fields, representing a period of time, depending on the type of interval
ARRAY	A set-length and ordered collection of elements
MULTISET	A variable-length and unordered collection of elements
XML	Stores XML data

SQL Data Type Quick Reference

However, different databases offer different choices for the data type definition.

The following table shows some of the common names of data types between the various database platforms:

Data type	Access	SQLServer	Oracle	MySQL	PostgreSQL
boolean	Yes/No	Bit	Byte	N/A	Boolean
integer	Number (integer)	Int	Number	Int Integer	Int Integer
float	Number (single)	Float Real	Number	Float	Numeric
currency	Currency	Money	N/A	N/A	Money
string (fixed)	N/A	Char	Char	Char	Char
string (variable)	Text (<256) Memo (65k+)	Varchar	Varchar Varchar2	Varchar	Varchar
binary object	OLE Object Memo	Binary (fixed up to 8K) Varbinary (<8K) Image (<2GB)	Long Raw	Blob Text	Binary Varbinary

SQL Server Data Types

String types:

Data type	Description	Storage
char(n)	Fixed width character string. Maximum 8,000 characters	Defined width
varchar(n)	Variable width character string. Maximum 8,000 characters	2 bytes + number of chars
varchar(max)	Variable width character string. Maximum 1,073,741,824 characters	2 bytes + number of chars
text	Variable width character string. Maximum 2GB of text data	4 bytes + number of chars
nchar	Fixed width Unicode string. Maximum 4,000 characters	Defined width x 2
nvarchar	Variable width Unicode string. Maximum 4,000 characters
nvarchar(max)	Variable width Unicode string. Maximum 536,870,912 characters
ntext	Variable width Unicode string. Maximum 2GB of text data
bit	Allows 0, 1, or NULL
binary(n)	Fixed width binary string. Maximum 8,000 bytes
varbinary	Variable width binary string. Maximum 8,000 bytes
varbinary(max)	Variable width binary string. Maximum 2GB
image	Variable width binary string. Maximum 2GB

Number types:

Data type	Description	Storage
tinyint	Allows whole numbers from 0 to 255	1 byte
smallint	Allows whole numbers between -32,768 and 32,767	2 bytes
int	Allows whole numbers between -2,147,483,648 and 2,147,483,647	4 bytes
bigint	Allows whole numbers between -9,223,372,036,854,775,808 and 9,223,372,036,854,775,807	8 bytes
decimal(p,s)	Fixed precision and scale numbers. Allows numbers from -10^38 +1 to 10^38 –1. The p parameter indicates the maximum total number of digits that can be stored (both to the left and to the right of the decimal point). p must be a value from 1 to 38. Default is 18. The s parameter indicates the maximum number of digits stored to the right of the decimal point. s must be a value from 0 to p. Default value is 0	5-17 bytes
numeric(p,s)	Fixed precision and scale numbers. Allows numbers from -10^38 +1 to 10^38 –1. The p parameter indicates the maximum total number of digits that can be stored (both to the left and to the right of the decimal point). p must be a value from 1 to 38. Default is 18. The s parameter indicates the maximum number of digits stored to the right of the decimal point. s must be a value from 0 to p. Default value is 0	5-17 bytes
smallmoney	Monetary data from -214,748.3648 to 214,748.3647	4 bytes
money	Monetary data from -922,337,203,685,477.5808 to 922,337,203,685,477.5807	8 bytes
float(n)	Floating precision number data from -1.79E + 308 to 1.79E + 308. The n parameter indicates whether the field should hold 4 or 8 bytes. float(24) holds a 4-byte field and float(53) holds an 8-byte field. Default value of n is 53.	4 or 8 bytes
real	Floating precision number data from -3.40E + 38 to 3.40E + 38	4 bytes

Date types:

Data type	Description	Storage
datetime	From January 1, 1753 to December 31, 9999 with an accuracy of 3.33 milliseconds	8 bytes
datetime2	From January 1, 0001 to December 31, 9999 with an accuracy of 100 nanoseconds	6-8 bytes
smalldatetime	From January 1, 1900 to June 6, 2079 with an accuracy of 1 minute	4 bytes
date	Store a date only. From January 1, 0001 to December 31, 9999	3 bytes
time	Store a time only to an accuracy of 100 nanoseconds	3-5 bytes
datetimeoffset	The same as datetime2 with the addition of a time zone offset	8-10 bytes
timestamp	Stores a unique number that gets updated every time a row gets created or modified. The timestamp value is based upon an internal clock and does not correspond to real time. Each table may have only one timestamp variable

Other data types:

Data type	Description
sql_variant	Stores up to 8,000 bytes of data of various data types, except text, ntext, and timestamp
uniqueidentifier	Stores a globally unique identifier (GUID)
xml	Stores XML formatted data. Maximum 2GB
cursor	Stores a reference to a cursor used for database operations
table	Stores a result-set for later processing

SQL Functions

SQL has many built-in functions for performing calculations on data.

SQL Aggregate Functions

SQL aggregate functions return a single value, calculated from values in a column.

Useful aggregate functions:

AVG() - Returns the average value
COUNT() - Returns the number of rows
FIRST() - Returns the first value
LAST() - Returns the last value
MAX() - Returns the largest value
MIN() - Returns the smallest value
SUM() - Returns the sum

SQL Scalar functions

SQL scalar functions return a single value, based on the input value.

Useful scalar functions:

UCASE() - Converts a field to upper case
LCASE() - Converts a field to lower case
MID() - Extract characters from a text field
LEN() - Returns the length of a text field
ROUND() - Rounds a numeric field to the number of decimals specified
NOW() - Returns the current system date and time
FORMAT() - Formats how a field is to be displayed

SQL AVG() Function

The AVG() Function

The AVG() function returns the average value of a numeric column.

SQL AVG() Syntax

SELECT AVG(column_name) FROM table_name

Demo Database

In this tutorial, we will use the well-known Northwind sample database.

Below is a selection from the "Products" table:

ProductID	ProductName	SupplierID	CategoryID	Unit	Price
1	Chais	1	1	10 boxes x 20 bags	18
2	Chang	1	1	24 - 12 oz bottles	19
3	Aniseed Syrup	1	2	12 - 550 ml bottles	10
4	Chef Anton's Cajun Seasoning	2	2	48 - 6 oz jars	21.35
5	Chef Anton's Gumbo Mix	2	2	36 boxes	25

SQL AVG() Example

The following SQL statement gets the average value of the "Price" column from the "Products" table:

Example

SELECT AVG(Price) AS PriceAverage FROM Products;

The following SQL statement selects the "ProductName" and "Price" records that have an above average price:

Example

SELECT ProductName, Price FROM Products
WHERE Price>(SELECT AVG(Price) FROM Products);

SQL COUNT() Function

The COUNT() function returns the number of rows that matches a specified criteria.

SQL COUNT(column_name) Syntax

The COUNT(column_name) function returns the number of values (NULL values will not be counted) of the specified column:

SELECT COUNT(column_name) FROM table_name;

SQL COUNT(*) Syntax

The COUNT(*) function returns the number of records in a table:

SELECT COUNT(*) FROM table_name;

SQL COUNT(DISTINCT column_name) Syntax

The COUNT(DISTINCT column_name) function returns the number of distinct values of the specified column:

SELECT COUNT(DISTINCT column_name) FROM table_name;

Note: COUNT(DISTINCT) works with ORACLE and Microsoft SQL Server, but not with Microsoft Access.

Demo Database

In this tutorial we will use the well-known Northwind sample database.

Below is a selection from the "Orders" table:

OrderID	CustomerID	EmployeeID	OrderDate	ShipperID
10265	7	2	1996-07-25	1
10266	87	3	1996-07-26	3
10267	25	4	1996-07-29	1

SQL COUNT(column_name) Example

The following SQL statement counts the number of orders from "CustomerID"=7 from the "Orders" table:

Example

SELECT COUNT(CustomerID) AS OrdersFromCustomerID7 FROM Orders
WHERE CustomerID=7;

SQL COUNT(*) Example

The following SQL statement omits the WHERE clause and counts the total number of orders from the "Orders" table:

Example

SELECT COUNT(*) AS NumberOfOrders FROM Orders;

SQL COUNT(DISTINCT column_name) Example

Now we want to count the number of unique customers in the "Orders" table.

We use the following SQL statement:

SELECT COUNT(DISTINCT CustomerID) AS NumberOfCustomers FROM Orders;

The result-set will look like this:

NumberOfCustomers

which is the number of unique customers in the "Orders" table.

SQL MAX() Function

The MAX() Function

The MAX() function returns the largest value of the selected column.

SQL MAX() Syntax

SELECT MAX(column_name) FROM table_name;

Demo Database

In this tutorial, we will use the well-known Northwind sample database.

Below is a selection from the "Products" table:

ProductID	ProductName	SupplierID	CategoryID	Unit	Price
1	Chais	1	1	10 boxes x 20 bags	18
2	Chang	1	1	24 - 12 oz bottles	19
3	Aniseed Syrup	1	2	12 - 550 ml bottles	10
4	Chef Anton's Cajun Seasoning	2	2	48 - 6 oz jars	21.35
5	Chef Anton's Gumbo Mix	2	2	36 boxes	25

SQL MAX() Example

The following SQL statement gets the largest value of the "Price" column from the "Products" table:

Example

SELECT MAX(Price) AS HighestPrice FROM Products;

SQL MIN() Function

The MIN() Function

The MIN() function returns the smallest value of the selected column.

SQL MIN() Syntax

SELECT MIN(column_name) FROM table_name;

Demo Database

In this tutorial we will use the well-known Northwind sample database.

Below is a selection from the "Products" table:

ProductID	ProductName	SupplierID	CategoryID	Unit	Price
1	Chais	1	1	10 boxes x 20 bags	18
2	Chang	1	1	24 - 12 oz bottles	19
3	Aniseed Syrup	1	2	12 - 550 ml bottles	10
4	Chef Anton's Cajun Seasoning	2	2	48 - 6 oz jars	21.35
5	Chef Anton's Gumbo Mix	2	2	36 boxes	25

SQL MIN() Example

The following SQL statement gets the smallest value of the "Price" column from the "Products" table:

Example

SELECT MIN(Price) AS SmallestOrderPrice FROM Products;

SQL SUM() Function

The SUM() Function

The SUM() function returns the total sum of a numeric column.

SQL SUM() Syntax

SELECT SUM(column_name) FROM table_name;

Demo Database

In this tutorial we will use the well-known Northwind sample database.

Below is a selection from the "OrderDetails" table:

OrderDetailID	OrderID	ProductID	Quantity
1	10248	11	12
2	10248	42	10
3	10248	72	5
4	10249	14	9
5	10249	51	40

SQL SUM() Example

The following SQL statement finds the sum of all the "Quantity" fields for the "OrderDetails" table:

Example

SELECT SUM(Quantity) AS TotalItemsOrdered FROM OrderDetails;

SQL GROUP BY Statement

Aggregate functions often need an added GROUP BY statement.

The GROUP BY Statement

The GROUP BY statement is used in conjunction with the aggregate functions to group the result-set by one or more columns.

SQL GROUP BY Syntax

SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name;

Demo Database

In this tutorial we will use the well-known Northwind sample database.

Below is a selection from the "Orders" table:

OrderID	CustomerID	EmployeeID	OrderDate	ShipperID
10248	90	5	1996-07-04	3
10249	81	6	1996-07-05	1
10250	34	4	1996-07-08	2

And a selection from the "Shippers" table:

ShipperID	ShipperName	Phone
1	Speedy Express	(503) 555-9831
2	United Package	(503) 555-3199
3	Federal Shipping	(503) 555-9931

And a selection from the "Employees" table:

EmployeeID	LastName	FirstName	BirthDate	Photo	Notes
1	Davolio	Nancy	1968-12-08	EmpID1.pic	Education includes a BA....
2	Fuller	Andrew	1952-02-19	EmpID2.pic	Andrew received his BTS....
3	Leverling	Janet	1963-08-30	EmpID3.pic	Janet has a BS degree....

SQL GROUP BY Example

Now we want to find the number of orders sent by each shipper.

The following SQL statement counts as orders grouped by shippers:

Example

SELECT Shippers.ShipperName,COUNT(Orders.OrderID) AS NumberOfOrders FROM Orders
LEFT JOIN Shippers
ON Orders.ShipperID=Shippers.ShipperID
GROUP BY ShipperName;

GROUP BY More Than One Column

We can also use the GROUP BY statement on more than one column, like this:

Example

SELECT Shippers.ShipperName, Employees.LastName,
COUNT(Orders.OrderID) AS NumberOfOrders
FROM ((Orders
INNER JOIN Shippers
ON Orders.ShipperID=Shippers.ShipperID)
INNER JOIN Employees
ON Orders.EmployeeID=Employees.EmployeeID)
GROUP BY ShipperName,LastName;

SQL HAVING Clause

The HAVING Clause

The HAVING clause was added to SQL because the WHERE keyword could not be used with aggregate functions.

SQL HAVING Syntax

SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
HAVING aggregate_function(column_name) operator value;

Demo Database

In this tutorial, we will use the well-known Northwind sample database.

Below is a selection from the "Orders" table:

OrderID	CustomerID	EmployeeID	OrderDate	ShipperID
10248	90	5	1996-07-04	3
10249	81	6	1996-07-05	1
10250	34	4	1996-07-08	2

And a selection from the "Employees" table:

EmployeeID	LastName	FirstName	BirthDate	Photo	Notes
1	Davolio	Nancy	1968-12-08	EmpID1.pic	Education includes a BA...
2	Fuller	Andrew	1952-02-19	EmpID2.pic	Andrew received his BTS...
3	Levering	Janet	1963-08-30	EmpID3.pic	Janet has a BS degree...

SQL HAVING Example

Now we want to find if any of the customers have a total order of less than 2000.

We use the following SQL statement:

The following SQL statement finds if any of the employees have registered more than 10 orders:

Example

SELECT Employees.LastName, COUNT(Orders.OrderID) AS NumberOfOrders FROM (Orders
INNER JOIN Employees
ON Orders.EmployeeID=Employees.EmployeeID)
GROUP BY LastName
HAVING COUNT(Orders.OrderID) > 10;

Now we want to find the if the employees "Davolio" or "Fuller" has more than 25 orders

We add an ordinary WHERE clause to the SQL statement:

Example

SELECT Employees.LastName, COUNT(Orders.OrderID) AS NumberOfOrders FROM Orders
INNER JOIN Employees
ON Orders.EmployeeID=Employees.EmployeeID
WHERE LastName='Davolio' OR LastName='Fuller'
GROUP BY LastName
HAVING COUNT(Orders.OrderID) > 25;

SQL LEN() Function

The LEN() Function

The LEN() function returns the length of the value in a text field.

SQL LEN() Syntax

SELECT LEN(column_name) FROM table_name;

Demo Database

In this tutorial we will use the well-known Northwind sample database.

Below is a selection from the "Customers" table:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin	12209	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constitución 2222	México D.F.	05021	Mexico
3	Antonio Moreno Taquería	Antonio Moreno	Mataderos 2312	México D.F.	05023	Mexico
4	Around the Horn	Thomas Hardy	120 Hanover Sq.	London	WA1 1DP	UK
5	Berglunds snabbköp	Christina Berglund	Berguvsvägen 8	Luleå	S-958 22	Sweden

SQL LEN() Example

The following SQL statement selects the "CustomerName" and the length of the values in the "Address" column from the "Customers" table:

Example

SELECT CustomerName,LEN(Address) as LengthOfAddress
FROM Customers;

SQL Quick Reference

SQL Statement	Syntax
AND / OR	SELECT column_name(s) FROM table_name WHERE condition AND\|OR condition
ALTER TABLE	ALTER TABLE table_name ADD column_name datatype or ALTER TABLE table_name DROP COLUMN column_name
AS (alias)	SELECT column_name AS column_alias FROM table_name or SELECT column_name FROM table_name AS table_alias
BETWEEN	SELECT column_name(s) FROM table_name WHERE column_name BETWEEN value1 AND value2
CREATE DATABASE	CREATE DATABASE database_name
CREATE TABLE	CREATE TABLE table_name ( column_name1 data_type, column_name2 data_type, column_name2 data_type, ... )
CREATE INDEX	CREATE INDEX index_name ON table_name (column_name) or CREATE UNIQUE INDEX index_name ON table_name (column_name)

CREATE VIEW	CREATE VIEW view_name AS SELECT column_name(s) FROM table_name WHERE condition
DELETE	DELETE FROM table_name WHERE some_column=some_value or DELETE FROM table_name (Note: Deletes the entire table!!) DELETE * FROM table_name (Note: Deletes the entire table!!)
DROP DATABASE	DROP DATABASE database_name
DROP INDEX	DROP INDEX table_name.index_name (SQL Server) DROP INDEX index_name ON table_name (MS Access) DROP INDEX index_name (DB2/Oracle) ALTER TABLE table_name DROP INDEX index_name (MySQL)
DROP TABLE	DROP TABLE table_name
GROUP BY	SELECT column_name, aggregate_function(column_name) FROM table_name WHERE column_name operator value GROUP BY column_name
HAVING	SELECT column_name, aggregate_function(column_name) FROM table_name WHERE column_name operator value GROUP BY column_name HAVING aggregate_function(column_name) operator value
IN	SELECT column_name(s) FROM table_name WHERE column_name IN (value1,value2,..)
INSERT INTO	INSERT INTO table_name VALUES (value1, value2, value3,....) or INSERT INTO table_name (column1, column2, column3,...) VALUES (value1, value2, value3,....)

INNER JOIN	SELECT column_name(s) FROM table_name1 INNER JOIN table_name2 ON table_name1.column_name=table_name2.column_name
LEFT JOIN	SELECT column_name(s) FROM table_name1 LEFT JOIN table_name2 ON table_name1.column_name=table_name2.column_name
RIGHT JOIN	SELECT column_name(s) FROM table_name1 RIGHT JOIN table_name2 ON table_name1.column_name=table_name2.column_name
FULL JOIN	SELECT column_name(s) FROM table_name1 FULL JOIN table_name2 ON table_name1.column_name=table_name2.column_name
LIKE	SELECT column_name(s) FROM table_name WHERE column_name LIKE pattern
ORDER BY	SELECT column_name(s) FROM table_name ORDER BY column_name [ASC\|DESC]
SELECT	SELECT column_name(s) FROM table_name
SELECT *	SELECT * FROM table_name
SELECT DISTINCT	SELECT DISTINCT column_name(s) FROM table_name
SELECT INTO	SELECT * INTO new_table_name [IN externaldatabase] FROM old_table_name or SELECT column_name(s) INTO new_table_name [IN externaldatabase] FROM old_table_name
SELECT TOP	SELECT TOP number\|percent column_name(s) FROM table_name
TRUNCATE TABLE	TRUNCATE TABLE table_name
UNION	SELECT column_name(s) FROM table_name1 UNION SELECT column_name(s) FROM table_name2
UNION ALL	SELECT column_name(s) FROM table_name1 UNION ALL SELECT column_name(s) FROM table_name2
UPDATE	UPDATE table_name SET column1=value, column2=value,... WHERE some_column=some_value
WHERE	SELECT column_name(s) FROM table_name WHERE column_name operator value

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SQL(Structured Query Language) Tutorials

SQL SELECT Statement

The SQL SELECT Statement

SQL SELECT Syntax

Demo Database

SELECT Column Example

Example

SELECT * Example

Example

SQL SELECT DISTINCT Statement

The SQL SELECT DISTINCT Statement

SQL SELECT DISTINCT Syntax

Demo Database

SELECT DISTINCT Example

Example

SQL WHERE Claus

The SQL WHERE Clause

SQL WHERE Syntax

Demo Database

WHERE Clause Example

Example

Text Fields vs. Numeric Fields

Example

Operators in The WHERE Clause

SQL ORDER BY Keyword

The SQL ORDER BY Keyword

SQL ORDER BY Syntax

Demo Database

ORDER BY Example

Example

ORDER BY DESC Example

Example

ORDER BY Several Columns Example

Example

SQL INSERT INTO Statement

The SQL INSERT INTO Statement

SQL INSERT INTO Syntax

Demo Database

INSERT INTO Example

Insert Data Only in Specified Columns

SQL UPDATE Statement

The SQL UPDATE Statement

SQL UPDATE Syntax

Demo Database

SQL UPDATE Example

Update Warning!

SQL DELETE Statement

The SQL DELETE Statement

SQL DELETE Syntax

Demo Database

SQL DELETE Example

Delete All Data

SQL SELECT TOP Clause

The SQL SELECT TOP Clause

SQL Server / MS Access Syntax

SQL SELECT TOP Equivalent in MySQL and Oracle

MySQL Syntax

Example

Oracle Syntax

Example

Demo Database

SQL SELECT TOP Example

Example

SQL SELECT TOP PERCENT Example

Example

SQL LIKE Operator

The SQL LIKE Operator

SQL LIKE Syntax

Demo Database

SQL LIKE Operator Examples

Example

Example

Example

Example