Learn SQL

SQL is a standard computer language for accessing and manipulating databases.
What is SQL?
SQL stands for Structured Query Language
SQL allows you to access a database
SQL is an ANSI standard computer language
SQL can execute queries against a database
SQL can retrieve data from a database
SQL can insert new records in a database
SQL can delete records from a database
SQL can update records in a database
SQL is easy to learn
SQL is a Standard - BUT....
SQL is an ANSI (American National Standards Institute) standard computer language for accessing and manipulating database systems. SQL statements are used to retrieve and update data in a database. SQL works with database programs like MS Access, DB2, Informix, MS SQL Server, Oracle, Sybase, etc.
Unfortunately, there are many different versions of the SQL language, but to be in compliance with the ANSI standard, they must support the same major keywords in a similar manner (such as SELECT, UPDATE, DELETE, INSERT, WHERE, and others).
Note: Most of the SQL database programs also have their own proprietary extensions in addition to the SQL standard!
SQL 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.
Below is an example of a table called "Persons":
LastName
FirstName
Address
City
Hansen
Ola
Timoteivn 10
Sandnes
Svendson
Tove
Borgvn 23
Sandnes
Pettersen
Kari
Storgt 20
Stavanger
The table above contains three records (one for each person) and four columns (LastName, FirstName, Address, and City).
SQL Queries
With SQL, we can query a database and have a result set returned.
A query like this:
SELECT LastName FROM Persons
Gives a result set like this:
LastName
Hansen
Svendson
Pettersen
Note: Some database systems require a semicolon at the end of the SQL statement. We don't use the semicolon in our tutorials.
SQL Data Manipulation Language (DML)
SQL (Structured Query Language) is a syntax for executing queries. But the SQL language also includes a syntax to update, insert, and delete records.
These query and update commands together form the Data Manipulation Language (DML) part of SQL:
SELECT - extracts data from a database table
UPDATE - updates data in a database table
DELETE - deletes data from a database table
INSERT INTO - inserts new data into a database table
SQL Data Definition Language (DDL)
The Data Definition Language (DDL) part of SQL permits database tables to be created or deleted. We can also define indexes (keys), specify links between tables, and impose constraints between database tables.
The most important DDL statements in SQL are:
CREATE TABLE - creates a new database table
ALTER TABLE - alters (changes) a database table
DROP TABLE - deletes a database table
CREATE INDEX - creates an index (search key)
DROP INDEX - deletes an index
The SQL SELECT Statement
The SELECT statement is used to select data from a table. The tabular result is stored in a result table (called the result-set).
Syntax
SELECT column_name(s)FROM table_name
Note: SQL statements are not case sensitive. SELECT is the same as select.
SQL SELECT Example
To select the content of columns named "LastName" and "FirstName", from the database table called "Persons", use a SELECT statement like this:
SELECT LastName,FirstName FROM Persons
The database table "Persons":
LastName
FirstName
Address
City
Hansen
Ola
Timoteivn 10
Sandnes
Svendson
Tove
Borgvn 23
Sandnes
Pettersen
Kari
Storgt 20
Stavanger
The result
LastName
FirstName
Hansen
Ola
Svendson
Tove
Pettersen
Kari

Select All Columns
To select all columns from the "Persons" table, use a * symbol instead of column names, like this:
SELECT * FROM Persons
Result
LastName
FirstName
Address
City
Hansen
Ola
Timoteivn 10
Sandnes
Svendson
Tove
Borgvn 23
Sandnes
Pettersen
Kari
Storgt 20
Stavanger

The Result Set
The result from a SQL query is stored in a result-set. Most database software systems allow navigation of the result set with programming functions, like: Move-To-First-Record, Get-Record-Content, Move-To-Next-Record, etc.
Programming functions like these are not a part of this tutorial. To learn about accessing data with function calls, please visit our ADO tutorial.
Semicolon after SQL Statements?
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.
Some SQL tutorials end each SQL statement with a semicolon. Is this necessary? We are using MS Access and SQL Server 2000 and we do not have to put a semicolon after each SQL statement, but some database programs force you to use it.
The SELECT DISTINCT Statement
The DISTINCT keyword is used to return only distinct (different) values.
The SELECT statement returns information from table columns. But what if we only want to select distinct elements?
With SQL, all we need to do is to add a DISTINCT keyword to the SELECT statement:
Syntax
SELECT DISTINCT column_name(s)FROM table_name

Using the DISTINCT keyword
To select ALL values from the column named "Company" we use a SELECT statement like this:
SELECT Company FROM Orders
"Orders" table
Company
OrderNumber
Sega
3412
W3Schools
2312
Trio
4678
W3Schools
6798
Result
Company
Sega
W3Schools
Trio
W3Schools
Note that "W3Schools" is listed twice in the result-set.
To select only DIFFERENT values from the column named "Company" we use a SELECT DISTINCT statement like this:
SELECT DISTINCT Company FROM Orders
Result:
Company
Sega
W3Schools
Trio
Now "W3Schools" is listed only once in the result-set.
The WHERE clause is used to specify a selection criterion.
The WHERE Clause
To conditionally select data from a table, a WHERE clause can be added to the SELECT statement.
Syntax
SELECT column FROM tableWHERE column operator value
With the WHERE clause, the following operators can be used:
Operator
Description
=
Equal
<>
Not equal
>
Greater than
<
Less than
>=
Greater than or equal
<=
Less than or equal
BETWEEN
Between an inclusive range
LIKE
Search for a pattern
IN
If you know the exact value you want to return for at least one of the columns
Note: In some versions of SQL the <> operator may be written as !=
Using the WHERE Clause
To select only the persons living in the city "Sandnes", we add a WHERE clause to the SELECT statement:
SELECT * FROM PersonsWHERE City='Sandnes'
"Persons" table
LastName
FirstName
Address
City
Year
Hansen
Ola
Timoteivn 10
Sandnes
1951
Svendson
Tove
Borgvn 23
Sandnes
1978
Svendson
Stale
Kaivn 18
Sandnes
1980
Pettersen
Kari
Storgt 20
Stavanger
1960
Result
LastName
FirstName
Address
City
Year
Hansen
Ola
Timoteivn 10
Sandnes
1951
Svendson
Tove
Borgvn 23
Sandnes
1978
Svendson
Stale
Kaivn 18
Sandnes
1980

Using Quotes
Note that we have used single quotes around the conditional values in the examples.
SQL uses single quotes around text values (most database systems will also accept double quotes). Numeric values should not be enclosed in quotes.
For text values:
This is correct:SELECT * FROM Persons WHERE FirstName='Tove'This is wrong:SELECT * FROM Persons WHERE FirstName=Tove
For numeric values:
This is correct:SELECT * FROM Persons WHERE Year>1965This is wrong:SELECT * FROM Persons WHERE Year>'1965'

The LIKE Condition
The LIKE condition is used to specify a search for a pattern in a column.
Syntax
SELECT column FROM tableWHERE column LIKE pattern
A "%" sign can be used to define wildcards (missing letters in the pattern) both before and after the pattern.
Using LIKE
The following SQL statement will return persons with first names that start with an 'O':
SELECT * FROM PersonsWHERE FirstName LIKE 'O%'
The following SQL statement will return persons with first names that end with an 'a':
SELECT * FROM PersonsWHERE FirstName LIKE '%a'
The following SQL statement will return persons with first names that contain the pattern 'la':
SELECT * FROM PersonsWHERE FirstName LIKE '%la%'
The INSERT INTO Statement
The INSERT INTO statement is used to insert new rows into a table.
Syntax
INSERT INTO table_nameVALUES (value1, value2,....)
You can also specify the columns for which you want to insert data:
INSERT INTO table_name (column1, column2,...)VALUES (value1, value2,....)

Insert a New Row
This "Persons" table:
LastName
FirstName
Address
City
Pettersen
Kari
Storgt 20
Stavanger
And this SQL statement:
INSERT INTO Persons VALUES ('Hetland', 'Camilla', 'Hagabakka 24', 'Sandnes')
Will give this result:
LastName
FirstName
Address
City
Pettersen
Kari
Storgt 20
Stavanger
Hetland
Camilla
Hagabakka 24
Sandnes

Insert Data in Specified Columns
This "Persons" table:
LastName
FirstName
Address
City
Pettersen
Kari
Storgt 20
Stavanger
Hetland
Camilla
Hagabakka 24
Sandnes
And This SQL statement:
INSERT INTO Persons (LastName, Address)VALUES ('Rasmussen', 'Storgt 67')
Will give this result:
LastName
FirstName
Address
City
Pettersen
Kari
Storgt 20
Stavanger
Hetland
Camilla
Hagabakka 24
Sandnes
Rasmussen

Storgt 67

The Update Statement
The UPDATE statement is used to modify the data in a table.
Syntax
UPDATE table_nameSET column_name = new_valueWHERE column_name = some_value

Person:
LastName
FirstName
Address
City
Nilsen
Fred
Kirkegt 56
Stavanger
Rasmussen

Storgt 67


Update one Column in a Row
We want to add a first name to the person with a last name of "Rasmussen":
UPDATE Person SET FirstName = 'Nina'WHERE LastName = 'Rasmussen'
Result:
LastName
FirstName
Address
City
Nilsen
Fred
Kirkegt 56
Stavanger
Rasmussen
Nina
Storgt 67


Update several Columns in a Row
We want to change the address and add the name of the city:
UPDATE PersonSET Address = 'Stien 12', City = 'Stavanger'WHERE LastName = 'Rasmussen'
Result:
LastName
FirstName
Address
City
Nilsen
Fred
Kirkegt 56
Stavanger
Rasmussen
Nina
Stien 12
Stavanger
The DELETE Statement
The DELETE statement is used to delete rows in a table.
Syntax
DELETE FROM table_nameWHERE column_name = some_value

Person:
LastName
FirstName
Address
City
Nilsen
Fred
Kirkegt 56
Stavanger
Rasmussen
Nina
Stien 12
Stavanger

Delete a Row
"Nina Rasmussen" is going to be deleted:
DELETE FROM Person WHERE LastName = 'Rasmussen'
Result
LastName
FirstName
Address
City
Nilsen
Fred
Kirkegt 56
Stavanger

Delete All Rows
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_nameorDELETE * FROM table_name
Test your SQL Skills
On this page you can test your SQL skills.
We will use the Customers table in the Northwind database:
CompanyName
ContactName
Address
City
Alfreds Futterkiste
Maria Anders
Obere Str. 57
Berlin
Berglunds snabbköp
Christina Berglund
Berguvsvägen 8
Luleå
Centro comercial Moctezuma
Francisco Chang
Sierras de Granada 9993
México D.F.
Ernst Handel
Roland Mendel
Kirchgasse 6
Graz
FISSA Fabrica Inter. Salchichas S.A.
Diego Roel
C/ Moralzarzal, 86
Madrid
Galería del gastrónomo
Eduardo Saavedra
Rambla de Cataluña, 23
Barcelona
Island Trading
Helen Bennett
Garden House Crowther Way
Cowes
Königlich Essen
Philip Cramer
Maubelstr. 90
Brandenburg
Laughing Bacchus Wine Cellars
Yoshi Tannamuri
1900 Oak St.
Vancouver
Magazzini Alimentari Riuniti
Giovanni Rovelli
Via Ludovico il Moro 22
Bergamo
North/South
Simon Crowther
South House 300 Queensbridge
London
Paris spécialités
Marie Bertrand
265, boulevard Charonne
Paris
Rattlesnake Canyon Grocery
Paula Wilson
2817 Milton Dr.
Albuquerque
Simons bistro
Jytte Petersen
Vinbæltet 34
København
The Big Cheese
Liz Nixon
89 Jefferson Way Suite 2
Portland
Vaffeljernet
Palle Ibsen
Smagsløget 45
Århus
Wolski Zajazd
Zbyszek Piestrzeniewicz
ul. Filtrowa 68
Warszawa
To preserve space, the table above is a subset of the Customers table used in the example below.
Try it Yourself
To see how SQL works, you can copy the SQL statements below and paste them into the textarea, or you can make your own SQL statements.
SELECT * FROM customers

SELECT CompanyName, ContactName
FROM customers

SELECT * FROM customers
WHERE companyname LIKE 'a%'

SELECT CompanyName, ContactName
FROM customers
WHERE CompanyName > 'g'
AND ContactName > 'g'
Top of Form
SELECT * FROM customers
Bottom of Form