Underforespørgsel i SELECT-sætning (MySQL)

Du vil sandsynligvis bruge GROUP BY klausul i stedet for at gå med DISTINCT der, dvs.:

SELECT productId, Size,
SUM(CASE WHEN storeID IN ('BCN', 'BCN2') THEN stock ELSE 0 END) AS stockBCN,
SUM(CASE WHEN storeID = 'MAD' THEN stock ELSE 0 END) AS stockMAD
FROM stocks
GROUP BY productId, Size