LeetCode--SQL 查询：查找部门工资前三高的职员

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  |
+------------+----------+--------+

思路：前几名的一般使用group by + having

子查询：
SELECT b.`Name` AS Department, a.Salary, a.`Name` 
FROM employee a LEFT JOIN department b ON a.DepartmentId = b.Id 
WHERE a.Id in(
　　SELECT c.Id FROM employee c LEFT JOIN employee d 
　　on c.DepartmentId=d.DepartmentId and c.Salary < d.Salary
　　GROUP BY c.Id 
　　HAVING COUNT(c.Salary)<=2
)    
ORDER BY
a.departmentId, Salary DESC

表连接：
SELECT d.`Name` Department, a.`Name` Employee, a.salary Salary 
FROM employee a 
LEFT JOIN employee b ON a.departmentId = b.departmentId AND a.salary < b.salary 
LEFT JOIN department d ON a.departmentId = d.id
WHERE a.departmentId = d.id
GROUP BY a.id HAVING COUNT(DISTINCT b.salary) <=2
ORDER BY a.departmentId,a.salary DESC