Sql 查询 3 个表（计数）

Question

我有三个 table：S（员工）、C（客户）、O（运营）。下面是 tables 的截图。

如何打印每个城市的客户数量和居住在那里的员工数量？ 因此，table 将有 3 列：City、Count(Clinets)、Count(Stuff)

Answer 1

select city, sum(clients) as clients, sum(staff) as staff
from 
(
  select city, count() as clients, null as staff from clients group by city
  union all 
  select city, null, count() from staff group by city
) tmp
group by city