3个表上的SQL查询（计数）

Question

我有三个表：S（员工），C（客户），O（操作）。以下是表格的屏幕截图。

如何为每个城市打印客户数量和居住在那里的员工人数？ 因此，该表将有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