SELECT ss.quan FROM `stock` as ss JOIN `order_record` as r ON r.order_id = ss.order_id WHERE ss.from = 'c' AND r.from = 'c' AND r.status = 'ing' AND ss.prod_id = 120 GROUP BY ss.order_id 這是從數(shù)據(jù)庫(kù)撈出的： 這個(gè)數(shù)字是沒(méi)問(wèn)題的但是當(dāng)我改成這樣 SELECT sum(ss.quan) as num FROM `stock` as ss JOIN `order_record` as r ON r.order_id = ss.order_id WHERE ss.from = 'c' AND r.from = 'c' AND r.status = 'ing' AND ss.prod_id = 120 // GROUP BY ss.order_id 數(shù)字變成了 32 如果加入 GROUP BY ss.order_id 一樣是 32這很明顯有五個(gè)不同的 order_id 這可能各位會(huì)很難理解我只是想問(wèn)說(shuō)明明一開(kāi)始數(shù)字是對(duì)的當(dāng) ｓｕｍ 後就不對(duì)了當(dāng)使用 sum 要注意什麼事情？不能這樣直接 sum ？