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SQL Expressions


Logical Comparisons


Introduction



For your databases, you can create expressions that represent a combination of values, variables, and operators. To support expressions, Transact-SQL provides operators other than, or in addition to, those we saw in Lesson 2.

A comparison is a Boolean operation that produces a true or a false result, depending on the values on which the comparison is performed. A comparison is performed between two values of the same type; for example, you can compare two numbers, two characters, or the names of two cities. To support comparisons, Transact-SQL provides all necessary operators.

Conditional Statements


Introduction


IF a Condition is True



Probably the primary comparison you can perform on a statement is to find out whether it is true. This operation is performed using an IF statement in Transact-SQL. Its basic formula is:

IF Condition
Statement
When creating an IF statement, first make sure you provide a Condition expression that can be evaluated to produce true or false. To create this Condition, you can use variables and the logical comparison operator reviewed above.

When the interpreter executes this statement, it first examines the Condition to evaluate it to a true result. If the Condition produces true, then the interpreter executes the Statement. Here is an example:

DECLARE @DateHired As DateTime,
@CurrentDate As DateTime
SET @DateHired = '1996/10/04'
SET @CurrentDate = '2007/04/11'
IF @DateHired < @CurrentDate
PRINT 'You have the experience required for a new promotion in this job'
GO




source : http://blogs.msdn.com/sharepoint/default.aspx