带有 not in 子查询子句的 SQL 查询到 LINQ 查询答案

【问题标题】：Sql Query with not in clause of subquery to LINQ query带有 not in 子查询子句的 SQL 查询到 LINQ 查询
【发布时间】：2021-01-25 18:32:25
【问题描述】：

   select * from user_user_connection 
where userid_from = 3464 and 
userid_to not in
(select userid_from from 
user_user_connection 
where userid_to = 3464);

大家好，我是实体框架的新手。我正在尝试将此查询转换为无法理解如何使用 not in 子句编写子查询的 LINQ 查询。还有哪个更适合加入或包括？

【问题讨论】：

标签： c# entity-framework linq sql-to-linq-conversion

【解决方案1】：

这个

    from a in user_user_connection 
    join b in user_user_connection on 
        new {From=a.userid_to, To=a.userid_from} equals new {From=b.userid_from, To=b.userid_to} into c
    from d in c.DefaultIfEmpty()
    where d == null
    select a;

类似于

select a.*
from user_user_connection a
left join user_user_connection b on a.userid_to = b.userid_from and a.userid_from = b.userid_to
where b.userid_from is null

应该匹配你的 not in 查询。

如果你想要一个特定的userid_from，你可以在其中添加另一个

    from a in user_user_connection 
    where a.userid_from == 3464
    join b in user_user_connection on 
        new {From=a.userid_to, To=a.userid_from} equals new {From=b.userid_from, To=b.userid_to} into c
    from d in c.DefaultIfEmpty()
    where d == null
    select a;

【讨论】：

【解决方案2】：

可能是这样的：

var list = userConnection.Where(user => user.useridto == 3464).Select(user => user.userid_from).ToList();
var list2 = userConnection.Where(user => !list.Contains(user.userid_to));

可能可以改进，因为包含需要相当长的时间（也许将列表放入哈希集中？）

【讨论】：

【解决方案3】：

这应该可行：

var query = 
   from uc in ctx.user_user_connection
   where uc.userid_from = 3464 && 
     !(from uc2 in ctx.user_user_connection
      where uc2.userid_to = 3464
      select uc2.userid_from).Contains(uc.userid_to)
   select uc;

或通过EXISTS

var query = 
   from uc in ctx.user_user_connection
   where uc.userid_from = 3464 && 
     !(from uc2 in ctx.user_user_connection
      where uc2.userid_to = 3464 && uc.userid_to == uc2.userid_from
      select uc2).Any()
   select uc;

Ov 通过LEFT JOIN

var query = 
   from uc in ctx.user_user_connection
   from uc2 in ctx.user_user_connection
     .Where(uc2 => uc2.userid_from == 3464 && uc.userid_to == uc2.userid_from)
     .DefaultIfEmpty()
   where uc.userid_from = 3464 && uc2 == null
   select uc;

【讨论】：