MYSQL:JOIN使SUM加倍
问题描述：
这应该真的很容易.相信我，我已经研究了几个小时.我的查询:
This should be really easy. Believe me, I've been researching this for hours. My query:
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;
只有两个表，not_piece和part.
Only two tables, not_piece and part.
select qty_left
from not_piece
where pid='M-MP-007r8';
返回5.
由于part.name在部件表中出现两次(没关系)，所以总和为10，而不是5.
Since the part.name appears twice in the parts table (that's fine), the sum is 10, not 5.
如何在不加总的情况下加入此联接?
How do I make this join without doubling the sum?
谢谢.
答
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