LINQ 中的 C# 外部应用

Posted 2023-02-24

【中文标题】LINQ 中的 C# 外部应用

【英文标题】：C# Outer Apply in LINQ

【发布时间】：2011-03-02 03:47:34

【问题描述】：

如何在 LINQ 中实现 Outer Apply？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

【参考方案1】：

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
;

【参考方案2】：

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

【参考方案3】：

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