Tuesday, September 27, 2011

SQL WHERE Clause

The WHERE clause is used to filter records.

The WHERE clause is used to extract only those records that fulfill a specified criterion.

SQL WHERE Syntax

SELECT column_name(s)
FROM table_name
WHERE column_name operator value

WHERE Clause Example

The "Persons" table:

P_Id LastName FirstName Address City
1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

Now we want to select only the persons living in the city "Sandnes" from the table above.

We use the following SELECT statement:


SELECT * FROM Persons


WHERE City='Sandnes'

The result-set will look like this:

P_Id LastName FirstName Address City
1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes




Quotes Around Text Fields


SQL uses single quotes around text values (most database systems will also accept double quotes).


Although, 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'




Operators Allowed in the WHERE Clause

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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