【leetcode Database】185. Department Top Three Salaries

题目：

The Employee table holds all employees. Every employee has an Id, and there is also a column for the department Id.

+----+-------+--------+--------------+| Id | Name  | Salary | DepartmentId |+----+-------+--------+--------------+| 1  | Joe   | 70000  | 1            || 2  | Henry | 80000  | 2            || 3  | Sam   | 60000  | 2            || 4  | Max   | 90000  | 1            || 5  | Janet | 69000  | 1            || 6  | Randy | 85000  | 1            |+----+-------+--------+--------------+

The Department table holds all departments of the company.

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

Write a SQL query to find employees who earn the top three salaries in each of the department. For the above tables, your SQL query should return the following rows.

+------------+----------+--------+| Department | Employee | Salary |+------------+----------+--------+| IT         | Max      | 90000  || IT         | Randy    | 85000  || IT         | Joe      | 70000  || Sales      | Henry    | 80000  || Sales      | Sam      | 60000  |+------------+----------+--------+

解析：

首先，按照部门Id和Salary对Employee表进行排序；之后，添加rank字段，为工资顺序序号。最后，再和Department表连接，得到最终数据。代码如下：

# Write your MySQL query statement belowSELECT d.Name AS Department, e.Name AS Employee,e.Salary AS Salary FROM  (SELECT Name,Salary,DepartmentId, @Rank:=IF(@DepId != DepartmentId,1,IF(@Sal = Salary,@Rank,@Rank+1)) AS RANK, @DepId:=DepartmentId, @sal:= Salary FROM  (SELECT Name,Salary,DepartmentId FROM Employee ORDER BY DepartmentId,Salary DESC) t, (SELECT @Rank := 0, @DepId := NULL, @sal := NULL) r) e  INNER JOIN Department d ON e.DepartmentId = d.Id WHERE e.Rank <=3;