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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


WHILE



To examine a condition and evaluate it before taking action, you can use the WHILE operator. The basic formula of this statement is:

WHILE Expression
Statement
When implementing this statement, first provide an Expression after the WHILE keyword. The Expression must produce a true or a false result. If the Expression is true, then the interpreter executes the Statement. After executing the Statement, the Expression is checked again. AS LONG AS the Expression is true, it will keep executing the Statement. When or once the Expression becomes false, it stops executing the Statement. This scenario can be illustrated as follows:


Here is an example:

DECLARE @Number As int

WHILE @Number < 5
SELECT @Number AS Number
GO
To effectively execute a while condition, you should make sure you provide a mechanism for the interpreter to get a referenced value for the condition, variable, or expression being checked. This is sometimes in the form of a variable being initialized although it could be some other expression. Such a while condition could be illustrated as follows

This time, the statement would be implemented as follows:

DECLARE @Number As int
SET @Number = 1
WHILE @Number < 5
BEGIN
SELECT @Number AS Number
SET @Number = @Number + 1
END
GO



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