Are you simply looking for GREATEST?
SELECT
t.*,
GREATEST(T.Discount_Product, T.Discount_Code, T.Discount_Newsletter) as 'maxval'
FROM Temp T;
However GREATEST Returns NULL when a value is NULL, so you might want to care about this, too. For instance:
SELECT
t.*,
GREATEST
(
coalesce(T.Discount_Product,0),
coalesce(T.Discount_Code, 0),
coalesce(T.Discount_Newsletter, 0)
) as 'maxval'
FROM Temp T;
EDIT: In case GREATEST is not available in your dbms you can use a case expression.
SELECT
t.*,
CASE
WHEN coalesce(T.Discount_Product, 0) > coalesce(T.Discount_Code, 0)
AND coalesce(T.Discount_Product, 0) > coalesce(T.Discount_Newsletter, 0)
THEN coalesce(T.Discount_Product, 0)
WHEN coalesce(T.Discount_Code, 0) > coalesce(T.Discount_Product, 0)
AND coalesce(T.Discount_Code, 0) > coalesce(T.Discount_Newsletter, 0)
THEN coalesce(T.Discount_Code, 0)
ELSE coalesce(T.Discount_Newsletter, 0)
END
FROM Temp T;
EDIT: To get your own statement syntactically correct, do:
SELECT
t.*,
(
select MAX(Value)
FROM
(
SELECT T.Discount_Product AS Value
UNION ALL
SELECT T.Discount_Code
UNION ALL
SELECT T.Discount_Newsletter
) dummy -- T-SQL requires a name for such sub-queries
) as maxval
FROM Temp T;