Checking an input param if not Null and using it in where in SQL Server

Question

What is the best way to include an input param in the WHERE clause but exclude it if it is null?

There are a number of ways I believe, but I can\'t seem

Answer 1

This question really helped me with a similar issue that had a few of us scratching our heads for a bit. I only write it up in case somebody else tries the same approach and cannot figure out why it does not work.

I was trying to only evaluate a part of a multipart WHERE clause if the @Parameter was not null. I tried to do this as below but always had no rows returned if @Parameter was null.

DECLARE @Parameter int = null;

SELECT  *  FROM  TABLE
WHERE   [AlternateID] is not null 
        AND (@Parameter is not null AND [AlternateID] = @Parameter)

I incorrectly thought that (@Parameter is not null AND [AlternateID] = @Parameter) would simply not form part of the full WHERE clause is @Parameter was null. However it was making the entire WHERE clause return false. The remedy was to add an OR 1=1 as below:

WHERE   [AlternateID] is not null 
        AND (@Parameter is not null AND [AlternateID] = @Parameter OR 1=1)

Of course the approach outlined by Ali (not enough reputation to upvote) solves this more efficiently.

WHERE   [AlternateID] is not null 
        AND [Partner_Customer_ID] = ISNULL(@Parameter, [Partner_Customer_ID])