The SQL WHERE clause is used to select data conditionally, by adding it to already existing SQL 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 | DOB | Phone | |
| John Smith | John.Smith@yahoo.com | 2/4/1968 | 626 222-2222 |
| Roman Regmi | romanregmi@phpfresher.com | 4/4/1974 | 323 455-4545 |
| amit | amit@herowndomain.org | 5/24/1978 | 416 323-3232 |
| Nimesh | nimesh@gmail.com | 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 SQL 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’