在mysql中选择每个组的顶行

Question

我有一个包含两列的表 - 区域和专业

region   |  profession
India    |  IT
India    |  IT
US       |  HR
US       |  HR
India    |  HR

我希望在每个地区展示最受欢迎的专业。输出应该只是

Region | Profession
India  | IT
US     | HR

Answer 1

首先，获得每组count的结果。然后你需要为每个组建立一个订单 - mysql可以使用user-defined variables执行此操作。

以下是一个例子：

select region, profession
from (
  select region, profession, 
    @rn:=if(@oldregion=region,@rn+1,0) rn,
    @oldregion:=region
  from (
    select region, profession, count(*) cnt
    from yourtable
    group by region, profession) t, (select @rn:=0, @oldregion='') t2
  order by region, cnt desc) t3
where rn = 0

• SQL Fiddle Demo

这是使用相关子查询的另一种解决方案：

select distinct region, (
    select profession
    from yourtable t2
    where t.region = t2.region
    group by region, profession
    order by count(*) desc
    limit 1) as profession
from yourtable t

• More Fiddle