Question

我试图找出某一行是否具有组中的最大值。这是一个非常简单的例子：

数据

VoteCount   LocationId  UserId
3           1           1
4           1           2
3           2           2
4           2           1

伪查询

select 
    LocationId,
    sum(case 
          when UserId  = 1  /* and has max vote count*/ 
            then 1 else 0 
        end) as IsUser1Winner,
    sum(case 
          when UserId  = 2  /* and has max vote count*/ 
            then 1 else 0 
        end) as IsUser2Winner
from LocationVote
group by LocationID

它应该返回：

LocationId  IsUser1Winner   IsUser2Winner
1           0           1
2           1           1

我也找不到在这里生成动态列名的方法。编写此查询的最简单方法是什么？

Answer 1

您也可以使用Case声明

执行此操作

WITH CTE as 
    (SELECT 
        MAX(VoteCount) max_votes
        , LocationId
    FROM LocationResult
    group by LocationId
    )
    SELECT 
        A.LocationId 
        , Case When UserId=1
            THEN 1
            ELSE 0
            END IsUser1Winner   
        , Case when UserId=2
            THEn 1
            ELSE 0
            END IsUser2Winner   
    from LocationResult A
    inner join 
    CTE B
    on A.VoteCount = B.max_votes
    and A.LocationId = B.LocationId

Answer 2

试试这个：
select *
from table t
cross apply (
select max(votes) max_value
from table ref
where ref.group = t.group
)votes
where votes.max_value = t.votes
但如果你的桌子很大并且没有专有的索引，那么性能可能会很差另一种方法是按组将最大值获取到表变量或临时表中，然后将其连接到原始表。

检查行是否具有组中的最大值

2 个答案: