Question

我有表a，b，c和d，其中：

There are 0 or more b rows for each a row
There are 0 or more c rows for each a row
There are 0 or more d rows for each a row

如果我尝试以下查询：

SELECT a.id, SUM(b.debit), SUM(c.credit), SUM(d.other)
FROM a
LEFT JOIN b on a.id = b.a_id
LEFT JOIN c on a.id = c.a_id
LEFT JOIN d on a.id = d.a_id
GROUP BY a.id

我注意到我创建了一个笛卡尔积，因此我的总和不正确（太大了）。

我看到还有其他SO问题和答案，但是我仍然没有理解如何在单个查询中完成我想要做的事情。在SQL中是否可以编写聚合以下所有数据的查询：

SELECT a.id, SUM(b.debit)
FROM a
LEFT JOIN b on a.id = b.a_id
GROUP BY a.id

SELECT a.id, SUM(c.credit)
FROM a
LEFT JOIN c on a.id = c.a_id
GROUP BY a.id

SELECT a.id, SUM(d.other)
FROM a
LEFT JOIN d on a.id = d.a_id
GROUP BY a.id

在一个查询中？

Answer 1

您的分析是正确的。不相关JOIN创建笛卡尔积。

您必须单独进行总和，然后再进行最后添加。这在一个查询中是可行的，您有几个选项：

SELECT中的子请求：SELECT a.id, (SELECT SUM(b.debit) FROM b WHERE b.a_id = a.id) + ...
CROSS APPLY的查询类似于第一个项目符号SELECT a.id, b_sum + c_sum + d_sum
UNION ALL正如你所建议的那样，在SUM和GROUP BY之上。
LEFT JOIN与上述类似的子查询。

可能更多......各种解决方案的性能可能略有不同，具体取决于您要选择A中的行数。

Answer 2

SELECT a.ID, debit, credit, other
FROM a
LEFT JOIN (SELECT a_id, SUM(b.debit) as debit
           FROM b
          GROUP BY a_id) b  ON a.ID = b.a_id
LEFT JOIN (SELECT a_id, SUM(b.credit) as credit
           FROM c
          GROUP BY a_id) c  ON a.ID = c.a_id
LEFT JOIN (SELECT a_id, SUM(b.other) as other
           FROM d
          GROUP BY a_id) d  ON a.ID = d.a_id

Answer 3

也可以使用相关子查询来完成：

SELECT a.id
     , (SELECT SUM(debit) FROM b WHERE a.id = b.a_id)
     , (SELECT SUM(credit) FROM c WHERE a.id = c.a_id)
     , (SELECT SUM(other) FROM d WHERE a.id = d.a_id)
 FROM a

具有多个外连接的SQL求和

3 个答案: