SQL Server provides a mechanism for returning different values in a SELECT clause based on Boolean conditions: the CASE statement. This statement resembles Visual Basics Select Case statement.

The SQL CASE statement has WHEN, THEN, and ELSE clauses along with an END terminator. The syntax is:

CASE [expression]	WHEN [value | Boolean expression] THEN [return value]	[ELSE [return value]]END

The [expression] is optional and contains a table column or a variable. When you specify [expression] directly after the CASE, you must populate the [value] parameter in the WHEN clause:

DECLARE @TestVal intSET @TestVal = 3SELECT	CASE @TestVal		WHEN 1 THEN 'First'		WHEN 2 THEN 'Second'		WHEN 3 THEN 'Third'		ELSE 'Other'	END

SQL Server compares this value to the expression and when the values match, it returns the THEN clauses [return value]. If none of the WHEN clauses equates to true, SQL Server returns the [return value] in the optional ELSE clause. If the ELSE clause is omitted and no value is matched, NULL is returned.

If you dont specify [expression], you must include the [Boolean expression] in the WHEN clause. This can contain any valid Boolean expression SQL Server allows:

DECLARE @TestVal intSET @TestVal = 5SELECT	CASE		WHEN @TestVal <=3 THEN 'Top 3'		ELSE 'Other'	END