sql - MySQL 连接两个表从第二个表计数和求和

Question

我有拖车:

经销商
带有一些字段和主键 id

和
查询
具有以下字段
身份证
经销商_id
成本

每个经销商都有几项要查询的项目，我必须计算它们并总结成本。现在我只有这个语句的计数:

SELECT a.*, Count(b.id) as counttotal
FROM dealers a
LEFT JOIN inquiries b on a.id=b.dealer_id
GROUP BY a.id
ORDER BY name ASC

但我不知道如何为每个经销商计算表 b 的成本。有人可以帮忙吗？提前致谢

Answer 1

您可以使用两个子查询:

SELECT  a.*
      , (SELECT Count(b.id) FROM inquiries I1 WHERE I1.dealer_id = a.id) as counttotal
      , (SELECT SUM(b.cost) FROM inquiries I2 WHERE I2.dealer_id = a.id) as turnover
FROM dealers a
ORDER BY name ASC

要么

SELECT  a.*
     , COALESCE(T.counttotal, 0) as counttotal   -- use coalesce or equiv. to turn NULLs to 0
     , COALESCE(T.turnover, 0) as turnover       -- use coalesce or equiv. to turn NULLs to 0
 FROM dealers a
 LEFT OUTER JOIN (SELECT a.id, Count(b.id) as counttotal, SUM(b.cost) as turnover
               FROM dealers a1 
               INNER JOIN inquiries b ON a1.id = b.dealer_id
              GROUP BY a.id) T
         ON a.id = T.id
ORDER BY a.name