SQL - WHERE

The WHERE clause is used to select data conditionally, by adding it to already existing SELECT query. We are going to use the Customers table from the previous chapter, to illustrate the use of the SQL WHERE command.

Table: Customers

FirstName	LastName	Email	DOB	Phone
John	Smith	John.Smith@yahoo.com	2/4/1968	626 222-2222
Steven	Goldfish	goldfish@fishhere.net	4/4/1974	323 455-4545
Paula	Brown	pb@herowndomain.org	5/24/1978	416 323-3232
James	Smith	jim@supergig.co.uk	20/10/1980	416 323-8888

If we want to select all customers from our database table, having last name 'Smith' we need to use the following SQL syntax:

SELECT * FROM Customers WHERE LastName = 'Smith'

The result of the SQL expression above will be the following:

FirstName	LastName	Email	DOB	Phone
John	Smith	John.Smith@yahoo.com	2/4/1968	626 222-2222
James	Smith	jim@supergig.co.uk	20/10/1980	416 323-8888

In this simple SQL query we used the "=" (Equal) operator in our WHERE criteria: LastName = 'Smith'

But we can use any of the following comparison operators in conjunction with the WHERE clause: <> (Not Equal)

SELECT * FROM Customers WHERE LastName <> 'Smith'

> (Greater than)

SELECT * FROM Customers WHERE DOB > '1/1/1970'

>= (Greater or Equal)

SELECT * FROM Customers WHERE DOB >= '1/1/1970'

< (Less than)

SELECT * FROM Customers WHERE DOB < '1/1/1970'

<= (Less or Equal)

SELECT * FROM Customers WHERE DOB =< '1/1/1970'

LIKE (similar to)

SELECT * FROM Customers WHERE Phone LIKE '626%'

Note the LIKE syntax is different with the different RDBMS (SQL Server syntax used above). Check the LIKE article for more details.

Between (Defines a range)

SELECT * FROM Customers WHERE DOB BETWEEN '1/1/1970' AND '1/1/1975'