MYSQL：JOIN make SUM double

Question

這應該很容易。相信我，我一直在研究這個好幾個小時。我查詢：

SELECT not_piece.pid, part.name AS 'Part Name', SUM(qty_left) AS 'In Stock' 
FROM not_piece 
JOIN part ON not_piece.pid = part.pid 
GROUP BY part.name; 

只有兩張桌子，not_piece和零件。

select qty_left 
from not_piece 
where pid='M-MP-007r8'; 

返回5

由於part.name在零件表中出現兩次（這很好），總和爲10，而不是5

如何使這個連接，而不加倍總和？

謝謝。

Answer 1

SELECT t.pid, p.name AS 'Part Name', t.InStock AS 'In Stock' 
    FROM (SELECT pid, SUM(qty_left) AS InStock 
       FROM not_piece 
       GROUP BY pid) t 
     INNER JOIN part p 
      ON t.pid = p.pid 

Answer 2

有了您GROUP BY，嘗試通過您的主鍵分組，PID：

SELECT not_piece.pid, part.name AS 'Part Name', SUM(qty_left) AS 'In Stock' 
FROM not_piece 
JOIN part ON not_piece.pid = part.pid 
GROUP BY part.pid;