185. Department Top Three Salaries

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

比184题复杂一点，求部门工资排名前三位的员工信息。

贴答案：

select d.Name Department,e1.Name Employee,e1.Salary Salary from
Employee e1,Department d where e1.DepartmentId = d.Id and 3 >
(select Count(Distinct e2.salary) from Employee e2 where e2.DepartmentId = d.Id and e2.Salary > e1.Salary)
Order BY d.Name,e1.salary desc 这道题关键在于前三位的理解，即忽略重复，（若有并排情况算作1位。）前三位有两种方法，一种用group by语句然后limit offset来显示所求。一种是直接查询，限定条件为比其薪水高的员工低于三个（不包含三个）。仍然用184题所使用的循环思想。e1.salary是改变着的，只要查出比他大的e2.salary的个数即可（并对e2.salary去重）如果误写成对e1.salary去重则限定条件无意义，因为对子查询来说，salary的个数永远是一个。
用group by 的解法

SELECT d.Name AS Department, e.Name AS Employee, e.Salary FROM
(SELECT e1.Name, e1.Salary, e1.DepartmentId FROM Employee e1 JOIN Employee e2
ON e1.DepartmentId = e2.DepartmentId AND e1.Salary <= e2.Salary GROUP BY e1.Id
HAVING COUNT(DISTINCT e2.Salary) <= 3) e JOIN Department d ON e.DepartmentId = d.Id
ORDER BY d.Name, e.Salary DESC;

解释：

Employee表自连接，限定条件e1.salary > e2.salary的目的是用数量表示排名,e1.salary<=e2.salary筛选出来的列在以e1.id分组求Count（DISTINCT e2.salary）筛选出的值就是前三名。最后在与部门表关联得出结果。