如何将聚合查询与不同联接结合在一起？

Question

我一直在研究this SO post和this jooq.org post，试图弄清楚如何在 MySQL 中进行合并聚合，但是运气不佳。

这是我的2个查询：

select a.IsTestAgency, a.ID, a.AgencyName, a.CreateDate, count(*) AS Agents
from Users u
join Agencies a
on u.AgencyID = a.ID
group by a.IsTestAgency, a.ID, a.AgencyName, a.CreateDate
order by a.IsTestAgency, a.AgencyName;

结果：

和：

select a.IsTestAgency, a.ID, a.AgencyName, a.CreateDate, count(*) AS Certs
from Certificates c
join Agencies a
on c.AgencyID = a.ID
group by a.IsTestAgency, a.ID, a.AgencyName, a.CreateDate
order by a.IsTestAgency, a.AgencyName;

结果：

您可以看到列和列的数据类型匹配。我想将它们组合成一个查询，并同时显示Agents计数和Certs计数，因为这是结果集中唯一的两个列值。

如何完成？

Answer 1

这是您想要的吗？

select a.*, 
       (select count(*)
        from users u
        where u.AgencyID = a.ID
       ) as users_count,
       (select count(*)
        from Certificates c
        where c.AgencyID = a.ID
       ) as certificates_count
from Agencies a
order by a.IsTestAgency, a.AgencyName;

Answer 2

您可以通过JOIN到COUNT s个表中来做到这一点：

select a.IsTestAgency, a.ID, a.AgencyName, a.CreateDate, u.Agents, c.Certs
from Agencies a
join (select AgencyID, COUNT(*) as Agents from Users group by AgencyID) u on u.AgencyID = a.ID
join (select AgencyID, COUNT(*) as Certs from Certficates group by AgencyID) c on c.AgencyID = a.ID
order by a.IsTestAgency, a.AgencyName;

这消除了在顶部查询中使用group by的需要，并且省去了对输出的每一行进行两次子查询计数的过程。