SQL The WHERE Clause

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 table
WHERE column operator value

With the WHERE clause, the following operators can be used:

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 Persons
WHERE City='Sandnes'

"Persons" table

Result

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'

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.

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%'