题目

Employee 表包含所有员工信息，每个员工有其对应的工号Id，姓名 Name，工资 Salary 和部门编号 DepartmentId 。

Id	Name	Salary	DepartmentId
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

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。

题解

SELECT `d`.`Name` AS `Department`,
       `t`.`Name` AS `Employee`,
       `t`.`Salary`
FROM (
         SELECT `t`.*,
                CAST(
                  (
                      CASE
                          WHEN `t`.`DepartmentId` = @`preDeptId`
                              THEN (
                              -- 工资是否一样
                              IF(
                                    @`preSalary` <> `t`.`Salary` AND (@`preSalary` := `t`.`Salary`) IS NOT NULL,
                                    @`curRank` := @`curRank` + 1,
                                    @`curRank`
                                  )
                              )
                          WHEN (@`preDeptId` := `t`.`DepartmentId`) IS NOT NULL
                              AND (@`preSalary` := `t`.`Salary`) IS NOT NULL
                              THEN @`curRank` := 1
                      END
                      ) AS SIGNED
                    ) AS `Rank`
         FROM (SELECT *
               FROM `Employee`
               ORDER BY `DepartmentId`, `Salary` DESC) `t`,
              (SELECT @`preDeptId` := NULL, @`preSalary` := NULL, @`curRank` := 0) `r`
     ) `t`
         INNER JOIN `Department` `d` ON `t`.`DepartmentId` = `d`.`Id`
WHERE `t`.`Rank` <= 3