題目：
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，按照部門Id和Salary對Employee表進行排序；之後，添加rank字段，爲工資順序序號。最後，再和Department表連接，得到最終數據。代碼如下：
# Write your MySQL query statement below
SELECT 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;