Hi!
This comment is totally OT to your original question relating to code erroneously producing nulls.
An alternative approach you may find interesting is to define IN_OUT as:
IN_OUT smallint default 1 check(value in (1,-1))
This can simplify the code a little when you want the value appropriately signed:
SELECT QUANTITY * IN_OUT AS REAL_QUANTITY FROM ...
Cheers,
John

> From: dinol.softedge <dinol@...> [snip]
> I have the following statement to determine whether stock is coming in
> or going out and assign the correct sign (+ or -) to a column called
> real_quantity. [snip]
> SELECT
> [snip]
> CASE
> WHEN IN_OUT = 'IN' THEN QUANTITY
> WHEN IN_OUT = 'OUT' THEN QUANTITY * -1
> END AS REAL_QUANTITY
> FROM
> TBL_INV_TRANS
> [snip]