MYSQL：JOIN使SUM加倍

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;

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

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

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;