Question

I found this SQL function that iterates over the table and it finds the ParentID the code works except when I put a where clause in it doesn’t work on the newly created column. If I don’t use the where clause it produces Expr1 as the root of the tree which is what I want but I can’t put a condition on it.

Can someone help me fix this problem?

ALTER FUNCTION dbo.fn_Root
(
    @PostID int
)
RETURNS int
AS 
BEGIN
DECLARE @R int

SELECT @R = CASE WHEN ParentPostID = 0 THEN PostID
                ELSE dbo.fn_Root(ParentPostID)
            END
        FROM Post
        WHERE PostID = @PostID

RETURN @R
END


SELECT PostID, Title, dbo.fn_Root(PostID) AS Expr1
FROM Post WHERE Expr1 = 5

Answer 1

Another possible workaround: you can put the calculated value in the WHERE clause again:

SELECT PostID, Title, dbo.fn_Root(PostID) AS Expr1
FROM Post 
WHERE dbo.fn_Root(PostID) = 5