Question

I am trying to put IF..ELSE in WHERE clause as below but getting error message

“Incorrect syntax near the keyword ‘IF’. Incorrect syntax near ‘)’.”

DECLARE @categoryID int

SET @categoryID = 0

SELECT * from SE_Auctions
  WHERE ItemCategoryID IN
  (
   IF @categoryID = 0
     SELECT CategoryID from SE_ItemCategory
   ELSE
     SELECT CategoryID from SE_ItemCategory 
    WHERE ParentID = @categoryID 
          OR CategoryID = @categoryID
  )

Answer 1

You cannot use an IF inside a subselect in that manner. Instead, use an AND to test for the value of the variable @categoryID in the WHERE clause. This will require two () groups with a logical OR between them.

WHERE 
-- First case, @categoryID = 0
(@categoryID = 0 AND ItemCategoryID IN (
  SELECT CategoryID from SE_ItemCategory
))
-- Other case, @categoryID <> 0
OR (@categoryID <> 0 AND ItemCategoryID IN (
  SELECT CategoryID from SE_ItemCategory 
    WHERE ParentID=@categoryID OR CategoryID=@categoryID
))