Agreggate Expression Error

Question

What could be wrong with the query below?
SELECT SUM(CASE WHEN (SELECT TOP 1 ISNULL(StartDate,'01-01-1900') FROM TestingTable ORDER BY StartDate Asc)<> '01-01-1900' THEN 1 ELSE 0 END) AS Testingvalue.
I get the error:
Cannot perform an aggregate function on an expression containing an aggregate or a subquery.
Thank you
Zionlite

Answer 1

The problem is you are trying to aggregate the results of a subquery (nothing to do with the case at all).
The way around this, is to put your subquery into the query itself, like this:
DECLARE @TestingTable TABLE (startDate date)
INSERT INTO @TestingTable (startDate)
VALUES (NULL)
SELECT SUM(CASE WHEN COALESCE(startDate,'1900-01-01') = '1900-01-01' THEN 0 ELSE 1 END)
FROM (SELECT TOP 1 startDate AS startDate
FROM @TestingTable
ORDER BY startDate asc) a
--alternative
SELECT SUM(CASE WHEN COALESCE(startDate,'1900-01-01') = '1900-01-01' THEN 0 ELSE 1 END)
FROM (SELECT MIN(startDate) AS startDate
FROM @TestingTable) a