185. Department Top Three Salaries
表结构
表：Employee
+----+-------+--------+--------------+
| 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 |
+----+-------+--------+--------------+
表：Department
+----+----------+
| Id | Name |
+----+----------+
| 1 | IT |
| 2 | Sales |
+----+----------+
问题描述
写个SQL查询，查找出各个部门薪水排前三的员工信息。查询结果如下表：
+------------+----------+--------+
| Department | Employee | Salary |
+------------+----------+--------+
| IT | Max | 90000 |
| IT | Randy | 85000 |
| IT | Joe | 70000 |
| Sales | Henry | 80000 |
| Sales | Sam | 60000 |
+------------+----------+--------+
解决方案
建立查询语句重点是要拆分业务逻辑，通过将Employee e1表中的薪水与Employee e2表中的数据比较统计出排名前列的数据。其中3可以改成任意数值，表示求出各个分组中排名前几的数据。
select d.Name Department ,e1.Name Employee ,e1.Salary
from Employee e1,Department d
where e1.DepartmentId =d.Id and
3>
(
select count(distinct(e2.Salary))
from Employee e2
where e1.DepartmentId=e2.DepartmentId and e1.Salary<e2.Salary
)
order by d.Id,e1.Salary desc
总结
拆分业务逻辑。