sql - 查询返回笛卡尔积 (Northwind)

Question

我在 oracle 中使用 northwind 数据库。

任务:取号在每个地区工作的员 worker 数。

结果:(地区名称，员 worker 数)

我正在尝试这个查询，但它返回笛卡尔积

select r.regiondescription, count(e.employeeid)
from employees e,
     employeeterritories et,
     territories t,
     region r 
where r.regionid = t.regionid
  and et.territoryid = t.territoryid
  and e.employeeid = et.employeeid
group by r.regiondescription;

问题:我的查询有什么问题？

Answer 1

我唯一能想到的是，您的一个表正在乘以结果，您应该改用 count(DISTINCT):

select r.regiondescription, count(distinct e.employeeid)
from employees e,
     employeeterritories et,
     territories t,
     region r 
where r.regionid = t.regionid
  and et.territoryid = t.territoryid
  and e.employeeid = et.employeeid
group by r.regiondescription;