您可能想利用GROUP BY子句，而不是在那里使用DISTINCT，即:

You probably want to leverage the GROUP BY clause instead of going with DISTINCT there, i.e.:

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