[英]Take each first element of the group by
How can I take each first element in the group with EF 5?如何使用 EF 5 获取组中的每个第一个元素?
var result = await context.SomeDbSet
.Where(...)
.GroupBy(x => new { x.SomeField, ... })
.Select(x => x.First())
.ToListAsync();
I am getting not supported exception.我得到不支持的异常。
How to correctly rewrite query?如何正确重写查询? Thanks.
谢谢。
You cannot do that with grouping.你不能通过分组来做到这一点。 SQL has a limitation - with
GROUP BY
you can select only grouping keys and aggregation result. SQL 有一个限制 - 使用
GROUP BY
你可以 select 只分组键和聚合结果。 This limitation for sure extended to LINQ to Entities
- after GroupBy
you can select only grouping keys and aggregation result.此限制肯定扩展到
LINQ to Entities
- 在GroupBy
之后,您可以 select 仅分组键和聚合结果。
Such result can be achieved by SQL and Window functions:这样的结果可以通过 SQL 和 Window 函数来实现:
SELECT
r.*,
FROM
(
SELECT
s.*,
ROW_NUMBER() OVER(PARTITION BY s.SomeField1, s.SomeField2 ORDER BY s.SomeDate) AS RN
FROM SomeDbSet s
WHERE ...
) r
WHERE r.RN = 1
For those who want to stay with LINQ, I propose extension (disclaimer: I'm extension creator) linq2db.EntityFrameworkCore对于那些想留在 LINQ 的人,我建议扩展(免责声明:我是扩展创建者) linq2db.EntityFrameworkCore
And you can write query above via LINQ您可以通过 LINQ 在上面编写查询
var rnQuery =
from s in context.SomeDbSet
where ...
select new
{
Data = s,
RN = Sql.Ext.RowNumber().Over()
.PartitionBy(s.SomeField1, s.SomeField2)
.OrderBy(s.SomeDate)
.ToValue()
}
var resultQuery = await rnQuery
.Where(r => r.RN == 1)
.Select(r => r.Data)
.ToLinqToDB();
var result = resultQuery.ToList();
// async variant may need ToListAsyncLinqToDB() call
// because of collision in async extension methods between EF Core and linq2db
var result = await resultQuery.ToListAsyncLinqToDB();
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.