Checking if String is NULL or EMPTY in SQL

I need to check in my Stored procedure if the information passed is null or empty so I can decided to insert the new value or keep the old. How do I accomplish this please in T-SQL. Thanks in advance.

IFISNULL(@.param)OR @.param =''THEN doSomething...

(Note the two apostrophes ' and ', not a quote mark!)

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Books online (help files that comes with SQL) is an amzing little app. Below is an example and the Syntax.

USE pubsGOSELECTAVG(ISNULL(price, $10.00))FROM titlesGOISNULL ( check_expression , replacement_value )

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Create Procedure mySpNameHere

@.InputValueHere VARCHAR(50) = NULL

AS

IF @.InputValue IS NULL OR @.InputValue = ''

/*Keep the old value*/

ELSE

/*Run the update statement here with your new value*/

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cheetahtech:

ISNULL ( check_expression , replacement_value )

Sorry. I got a blackout. Of course, ISNULL syntax is to be used in a query where you want to specify an alternative value, if the expression is NULL.

The correct way to check for NULL in a condition is IF @.Param IS NULL as rich freeman points out.