Question

这是数据

id       code       status
1          1          5
2          1          6
3          2          8
4          2          9
5          2          12
6          3          15
7          3          19
8          3          13

我需要的是：

id       code       status
2          1          6
5          2          12
8          3          19

换句话说，我需要在代码分组时使用max id的行。

例如，对于代码3，id为6,7,8，我需要8行。

等效的SQL查询将是

select * 
from t inner join (select max(id) maxId from t group by code) t1
on t.id = t1.maxId

状态列是相关的，它只是一个属性。

这样的linq查询是什么？

提前致谢。

Answer 1

from t in mytable
where !(from tt in mytable
        where tt.code == t.code &&
              tt.id > t.id
        select tt
       ).Any()
select t

Answer 2

Literal pick-a-winner

from row in rows
group row by row.code into g
let winner =
(
  from groupedrow in g
  order groupedrow  by groupedrow.id desc
  select groupedrow
).First()
select winner;

更传统的选择胜利者（没有机会进行自动组元素获取往返）

var subquery =
  from row in rows
  group row by row.code into g
  select new {Code = g.Key, Id = g.Max(row => row.Id)};

var query =
  from row in rows
  join key in subquery on new{row.Code, row.Id} == key
  select row;

与发布的sql完全匹配：

IQueryable<int> subquery =
  from row in rows
  group row by row.code into g
  select g.Max(row => row.Id);

var query =
  from row in rows
  join key in subquery on row.Id == key
  select row;

如何返回最大值为一列的行按另一列分组？

2 个答案: