SQL WHERE Clause

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The WHERE clause is used to specify a selection criterion.

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The WHERE Clause 

To conditionally select data from a table, a WHERE clause can be added to the SELECT statement.

Syntax

SELECT column FROM table
WHERE 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 !=

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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 Persons
WHERE 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

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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>1965
This is wrong:
SELECT * FROM Persons WHERE Year>'1965'

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The LIKE Condition

The LIKE condition is used to specify a search for a pattern in a column.

Syntax

SELECT column FROM table
WHERE column LIKE pattern

A "%" sign can be used to define wildcards (missing letters in the pattern) both before and after the pattern.

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Using LIKE

The following SQL statement will return persons with first names that start with an 'O':

SELECT * FROM Persons
WHERE FirstName LIKE 'O%'

The following SQL statement will return persons with first names that end with an 'a':

SELECT * FROM Persons
WHERE FirstName LIKE '%a'

The following SQL statement will return persons with first names that contain the pattern 'la':

SELECT * FROM Persons
WHERE FirstName LIKE '%la%'

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