1：数据表
2：问题：查询各个部门的最低工资的userid号
select a.* from
(select ROW_NUMBER() over(partition by dept order by salary asc) rn,userid,salary,dept from salary ) a
where rn=1
3：分析
partition by dept 是对dept进行分组，、
order by salary 分组后按salary进行排序
select ROW_NUMBER() over(partition by dept order by salary asc) rn,userid,salary,dept from salary
