c# - LINQ 中的 C# 外部应用

Question

如何在 LINQ 中实现外部应用？我有点问题。

这是我正在使用的 SQL 查询。

SELECT u.masterID
      ,u.user
      ,h.created
FROM dbo.Users u
OUTER APPLY (SELECT TOP 1 * FROM UserHistory h where h.masterID = u.masterID ORDER BY created DESC) h

Answer 1

from u in Users 
join UserHistory on u.masterID equals h.masterID into h 
select new {
  u.masterID,
  u.user,
  Created = h.Select(x => x.created).OrderByDescending(c => c).FirstOrDefault()
}

或者，通过关联：

from u in Users
let created = u.UserHistories.Select(x => x.created).OrderByDescending(c => c).FirstOrDefault()
select new
{
  u.masterID,
  u.user,
  Created = created
}

Answer 2

from u in Users
join UserHistory on u.masterID equals h.masterID into h
select new {u.masterID, u.user, h.created.OrderByDescending().First()}

Answer 3

Outer Apply 产生左外连接
的结果，查询应该是：

var q =
from u in db.Users
join h in db.UserHistory on u.masterID equals h.masterID into ps
from p in ps.DefaultIfEmpty()
select new { 
    masterID = u.masterID
    ,user = u.user
    ,created = ps.OrderByDescending(x=>x.created).FirstOrDefault()==null?null:ps.OrderByDescending(x=>x.created).First().created
};