作者:whisper
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Employee
表包含所有员工信息,每个员工有其对应的 Id, salary 和 department Id。+----+-------+--------+--------------+ | Id | Name | Salary | DepartmentId | +----+-------+--------+--------------+ | 1 | Joe | 70000 | 1 | | 2 | Henry | 80000 | 2 | | 3 | Sam | 60000 | 2 | | 4 | Max | 90000 | 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,找出每个部门工资最高的员工。例如,根据上述给定的表格,Max 在 IT 部门有最高工资,Henry 在 Sales 部门有最高工资。
+------------+----------+--------+ | Department | Employee | Salary | +------------+----------+--------+ | IT | Max | 90000 | | Sales | Henry | 80000 | +------------+----------+--------+
select d.Name Department, e.Name Employee, maxSalEmp.Salary
from Employee e, Department d,
(select DepartmentId, max(e2.Salary) Salary
from Employee e2
group by e2.DepartmentId) maxSalEmp
where d.Id = e.DepartmentId and e.DepartmentId = maxSalEmp.DepartmentId and e.Salary = maxSalEmp.Salary;
学习group by + 聚集函数求分组内的max值,三表连接
再看另一种较快的解法
select d.name as Department,a.name as Employee,a.Salary as Salary from Employee a inner join Department d on d.id=a.DepartmentId
where (a.salary,a.DepartmentId) in (select max(salary),DepartmentId from Employee e group by DepartmentId)
学习了inner join..on的用法,与两字段in的用法(第一次知道可以多字段in)
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。
select d.Name Department, e.Name Employee, e.Salary
from Employee e, Department d
where e.DepartmentId = d.Id
and (
select count(distinct e2.Salary) from Employee e2 where e2.Salary > e.Salary and e2.DepartmentId = e.DepartmentId
) < 3
order by e.DepartmentId, e.Salary desc;
学习了同一个表关联查询的知识,e1,e2相同表关联查询,查出高于某工资的不同工资小于3个的,说明某工资可能为最高,第二高,第三高,即前三的不同工资
再看另一种比较快的
select d.name as department,m.name as employee,salary
from
(select departmentid,Name,salary,
if(@pre_d=departmentid,if(@pre_s=salary,@cur_rank,@cur_rank:=@cur_rank+1),@cur_rank:=1) as ranking,
@pre_d:=departmentid,@pre_s:=salary
from employee,(select @pre_d:=null,@pre_s:=null,@cur_rank:=0) r
order by departmentid,salary desc) m join Department d on m.departmentid=d.id and m.ranking<=3
学习了变量的使用:在from中定义变量,在select中使用变量,还有对于复杂的判断,使用嵌套的if,多字段的order by及默认升序,使用desc降序,join等同inner join,也等同于用逗号","
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