why a variable is not set in this select statement?

Question

I have written the following stored procedure:

ALTER PROCEDURE [dbo].[sp_Accounts_ValidateLogin]
   @EmailAddress varchar(255),
   @Password varchar(20)
AS
   DECLARE @UserID int

   SELECT @UserID = UserID 
   FROM Accounts_Users 
   WHERE EmailAddress = @EmailAddress and Password = @Password

   IF @UserID != NULL   
      RETURN @UserID
   ELSE
      RETURN -1

When this procedure is executed it shows,

No rows affected.
(0 row(s) returned)
@RETURN_VALUE = -1

But whenever I rewrite the SELECT statement as,

SELECT UserID 
FROM Accounts_Users 
WHERE EmailAddress = @EmailAddress and Password = @Password

The result is:

UserID                                                                                                                                                                                                                                                           
---------------------------------------------------------------------------------------
3                                                                                                                                                                                                                                                                
No rows affected.
(1 row(s) returned)
@RETURN_VALUE = -1

My question is why the variable @UserID is not getting set inside the 1st select statement?

Answer 1

!= NULL will not work as you expect.

Make it:

IF @UserID IS NOT NULL 

The below SO question has a little information as to why, so I won't bother reposting it (it's basically because NULL is not an actual value):

Not equal <> != operator on NULL