SELECT a.品号 AS ph, a.对方中心 AS dfzx, a.减少数量 AS sl, b.bzsl AS bzsl
FROM (SELECT 对方中心, 品号, SUM(减少数量) AS 减少数量
        FROM 减少数量表
        WHERE 中心名称 = '铸造' AND 对方中心 <> '品号改动' AND 倒算 = 0
        GROUP BY 对方中心, 品号) AS a LEFT JOIN
          (SELECT 品号, SUM(sl) AS bzsl
         FROM (SELECT 品号, 减少数量 AS sl
                 FROM 减少数量表
                 WHERE 中心名称 = '铸造' AND 对方中心 <> '品号改动' AND 倒算 = 0
                 UNION
                 SELECT 品号, 结余数量 AS sl
                 FROM 结余数量表
                 WHERE 年份 = 2008 AND 月份 = 4 AND 中心名称 = '铸造')
         GROUP BY 品号) AS b ON a.品号 = b.品号

难道没人知道？

有活人吗？