我正在尝试对表'Personen'进行linq查询并加入表'Leners' 从表'Leners'我想要最新的项目(如果有的话)。
在SQL中,我会这样做:
select p.*, l.*
from dbo.Personen p
left outer join dbo.Leners l on l.LenerId =
(select top 1 LenerId
from dbo.Leners lt
where lt.PersoonId = p.PersoonId
order by lt.Vanaf desc)
如何使用linq执行此操作? 我想在lambda表达式中执行此操作,因为它取决于选择。
using (var db = new DbContext())
{
var query = from p in db.Personen
select p;
if (someSelection != null)
{
query = ?
}
}
这些是表格:( EF代码优先)
[Table("Personen")]
public class Persoon
{
[Key]
public int PersoonId { get; set; }
[StringLength(50)]
[Required]
public string Naam { get; set; }
}
[Table("Leners")]
public class Lener
{
public int LenerId { get; set; }
public int? PersoonId { get; set; }
[ForeignKey("PersoonId")]
public virtual Persoon Persoon { get; set; }
public DateTime Vanaf { get; set; }
}
答案 0 :(得分:3)
您的LINQ查询应如下所示:
var query2 = from p in dbo.Personen
let l = dbo.Leneren
.OrderByDescending(lt => lt.Vanaf)
.FirstOrDefault(lt => lt.PersoonId == p.PersoonId)
select new { p, l }
但是,您无法将其分配回query
变量!
var query = from p in db.Personen
select p;
query
为IQueryable<Person>
,因为您在此处选择Person
项。您无法将其重新指定为IQueryable<AnotherType>
。