问题
Employee 表包含所有员工信息，每个员工有其对应的 Id, salary 和 department Id。
| 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 |
准备
create database leecote184;
use leecote184;
create table employee (
id int(20),
name varchar(20),
salary int(20),
departmentid int(20)
);
insert into employee values
(1,"Joe",85000,1),
(2,"Henry",80000,2),
(3,"Sam",60000,2),
(4,"Max",90000,1),
(5,"Janet",69000,1),
(6,"Randy",85000,1),
(7,"Will",70000,1);
insert into employee values
(7,"sll",70000,1);
insert into employee values
(7,"sdll",90000,1);
create table department (
id int(20),
name varchar(20)
);
insert into department values
(1,"IT"),
(2,"Sales");
解题
SELECT
Department.name AS 'Department',
Employee.name AS 'Employee',
Salary
FROM
Employee
JOIN
Department ON Employee.DepartmentId = Department.Id
WHERE
(Employee.DepartmentId , Salary) IN
( SELECT
DepartmentId, MAX(Salary)
FROM
Employee
GROUP BY DepartmentId
)
;
延伸
编写一个SQL 查询，找出每个部门获得前三高工资的所有员工。例如，根据上述给定的表，查询结果应返回：
| Department | Employee | Salary |
|---|---|---|
| IT | Max | 90000 |
| IT | Randy | 85000 |
| IT | Joe | 85000 |
| IT | Will | 70000 |
| Sales | Henry | 80000 |
| Sales | Sam | 60000 |
解释：
IT 部门中，Max 获得了最高的工资，Randy 和 Joe 都拿到了第二高的工资，Will 的工资排第三。销售部门（Sales）只有两名员工，Henry 的工资最高，Sam 的工资排第二。
解题
#方法一：
select d.name as department,e.name as employee,salary from employee e join department d on e.departmentid = d.id
where (departmentid,salary) in (select departmentid,salary from
(
select *,@i:=if(@did=departmentid,if(@sa=salary,@i,@i+1),1) as rank,@sa:=salary,
@did:=departmentid from employee e,(select @i:=0,@sa:=-1,@did:=null) t
order by departmentid,salary desc) a where rank <= 3) order by department, salary desc ;
#方法二：
SELECT
d.Name AS 'Department', e1.Name AS 'Employee', e1.Salary
FROM Employee e1 JOIN Department d ON e1.DepartmentId = d.Id
WHERE 3 >
(SELECT COUNT(DISTINCT e2.Salary) FROM Employee e2
WHERE e2.Salary > e1.Salary AND e1.DepartmentId = e2.DepartmentId);
错误实例
select *,@i:=if(@did=departmentid,if(@sa=salary,@i,@i+1),1) as rank,@sa:=salary,
@did:=departmentid from employee e,(select @i:=0,@sa:=-1,@did:=null) t having rank<=3
order by departmentid,salary desc;
利用having 过滤 rank 2消失；
select *,@i:=if(@did=departmentid,@i+(@sa<>(@sa:=salary)),1) as rank,
@did:=departmentid from employee e,(select @i:=0,@sa:=-1,@did:=null) t having rank<=3
order by departmentid,salary desc;