Question

Is it possible to do a boolean compare in the when-expression on a case-expression within a SQL Server case statement. I have a SELECT statement that has logic similar to the following:

SELECT T1.ProductNumber,
    CASE
        WHEN (SELECT SUM(T2.Quantity) FROM Table2 AS T2 WHERE T2.Product=T1.Product) IS NULL
            THEN T1.Quantity
        WHEN (SELECT SUM(T2.Quantity) FROM Table2 AS T2 WHERE T2.Product=T1.Product) > T1.Quantity
            THEN T1.Quantity
        ELSE T1.Quantity - (SELECT SUM(T2.Quantity) FROM Table2 AS T2 WHERE T2.Product=T1.Product)
    END
FROM Table1 AS T1

This seems like a lot of duplicated lookups on Table2. If possible I’d like to do something like this:

SELECT T1.ProductNumber,
    CASE (SELECT SUM(T2.Quantity) FROM Table2 AS T2 WHERE T2.Product=T1.Product) as altQty
        WHEN altQty IS NULL
            THEN T1.Quantity
        WHEN altQty > T1.Quantity
            THEN T1.Quantity
        ELSE T1.Quantity - altQty
    END
FROM Table1 AS T1

Answer 1

You can’t reference the alias that way, but you can if you put it in a subquery / derived table.

SELECT ProductNumber, CASE WHEN altQty IS NULL
            THEN Quantity
        WHEN altQty > Quantity
            THEN Quantity
        ELSE Quantity - altQty END
FROM 
(
  SELECT T1.ProductNumber, T1.Quantity, 
    altQty = (SELECT SUM(T2.Quantity) FROM Table2 AS T2 WHERE T2.Product=T1.Product)
  FROM Table1 AS T1
) AS x;