题目如下：

Employee 表包含所有员工信息，每个员工有其对应的 Id, salary 和 departmentId。

+----+-------+--------+--------------+
| Id | Name  | Salary | DepartmentId |
+----+-------+--------+--------------+
| 1  | Joe   | 70000  | 1            |
| 2  | Henry | 80000  | 2            |
| 3  | Sam   | 60000  | 2            |
| 4  | Max   | 90000  | 1            |
+----+-------+--------+--------------+

Department 表包含公司所有部门的信息。

+----+----------+
| Id | Name     |
+----+----------+
| 1  | IT       |
| 2  | Sales    |
+----+----------+

编写一个 SQL 查询，找出每个部门工资最高的员工。例如，根据上述给定的表格，Max 在 IT 部门有最高工资，Henry 在 Sales 部门有最高工资。

+------------+----------+--------+
| Department | Employee | Salary |
+------------+----------+--------+
| IT         | Max      | 90000  |
| Sales      | Henry    | 80000  |
+------------+----------+--------+
解题思路

两种方式：

  • 使用子查询进行过滤
  • 使用join进行合并

使用子查询进行过滤（执行用时: 134 ms）

select d.Name Department,e.Name Employee,e.Salary Salary 
from Employee e,Department d 
where 
	e.DepartmentId=d.Id 
and 
	(e.DepartmentId,e.Salary) 			#返回两列
		in (
			select DepartmentId,max(Salary) from Employee group by DepartmentId
			)

返回结果：

{“headers”:[“Department”,“Employee”,“Salary”],
“values”:[[“IT”,“Jim”,90000],[“Sales”,“Henry”,80000],[“IT”,“Max”,90000]]}

使用join联表，再过滤（执行用时: 129 ms）

先使用join查询出每个部门最高工资的部门名称、薪水

select DepartmentId, 
        d.name as Name ,
        max(Salary) as Salary from Employee  e
    join Department d on d.Id = e.DepartmentId 
    group by DepartmentId

{“headers”:[“DepartmentId”,“Name”,“Salary”],
“values”:[[1,“IT”,90000],[2,“Sales”,80000]]}

再与Employee表进行联表查询，得出员工姓名

select 
t.Name as Department,
e.Name as Employee, 
e.Salary as Salary
from(
    select DepartmentId, 
        d.name as Name ,
        max(Salary) as Salary from Employee  e
    join Department d on d.Id = e.DepartmentId 
    group by DepartmentId) as t		# 表t查出 [[1,"IT",90000],[2,"Sales",80000]]
join Employee e
on t.DepartmentId = e.DepartmentId and e.Salary = t.Salary		# 连接条件（过滤）

得出正确结果：

{“headers”:[“Department”,“Employee”,“Salary”],
“values”:[[“IT”,“Jim”,90000],[“Sales”,“Henry”,80000],[“IT”,“Max”,90000]]}