SQL 分組內取前幾名的問題

查找 部門工資前三高的所有員工

比如這題，找每個部門的工資前三名，那么先在子查詢中用Employee和自己做連接，
連接條件是【部門相同但是工資比我高】，那么接下來按照having count(Salary) <= 2
來篩選的原理是：如果【跟我一個部門而且工資比我高的人數】不超過2個，那么我一定是部門工資前三，
這樣內層查詢可以查詢出所有符合要求的員工ID，接下來外層查詢就簡單了。```

# 內層查詢，找前三高工資員工
select e1.Id
from Employee e1 left join Employee e2
# 和我（e1）在同一部門的
on e1.DepartmentId=e2.DepartmentId
# 且工資比我高的
and e1.Salary<e2.Salary
# 不超過2個
group by Id
having count(distinct e2.Salary)<=2

# 外層查詢
select d.Name as Department,e.Name as Employee,e.Salary as Salary
from Employee e left join Department d
on  e.DepartmentId=d.Id
where e.Id in (
    "內層Id 結果集"
    )
    and e.DepartmentId in(
        select Id
        from Department
    )
order by d.Id asc,e.Salary desc

# 外層查詢
select d.Name as Department,e.Name as Employee,e.Salary as Salary
from Employee e left join Department d
on  e.DepartmentId=d.Id

where e.Id in (
    # 內層查詢，找前三高工資員工
select e1.Id
from Employee e1 left join Employee e2
# 和我（e1）在同一部門的
on e1.DepartmentId=e2.DepartmentId
# 且工資比我高的
and e1.Salary<e2.Salary
# 不超過2個
group by Id
having count(distinct e2.Salary)<=2
    )
    and e.DepartmentId in(
        select Id
        from Department
    )
order by d.Id asc,e.Salary desc