linq to sql:从同一个表中连接多个列

Question

如何通过Linq从同一个表中连接多个列？

例如:我已经有了......

join c in db.table2 on table2.ID equals table1.ID

我需要添加这个......

join d in db.table2 on table2.Country equals table1.Country 

Answer 1

这是我能够让它工作的唯一方法(在c#中).

var qry = from t1 in table1
          join t2 in table2
          on new {t1.ID,t1.Country} equals new {t2.ID,t2.Country}
          ...

Answer 2

您可以将查询放在Where子句中,而不是使用join运算符.

join操作符支持VB.NET中的多个子句,但不支持C#.

或者,您可以使用ANSI-82样式的"SQL"语法,例如:

from t1 in table1
from t2 in table1
where t1.x == t2.x
&& t1.y == t2.y

Answer 3

来自http://www.onedotnetway.com/linq-to-sql-join-on-multiple-conditions/

这两个表都有PostCode和CouncilCode作为公共字段.假设我们想要从ShoppingMall中检索所有记录,其中PostCode和HouseCode on House匹配.这要求我们使用两列进行连接.在LINQ中,可以使用匿名类型完成连接.这是一个例子.

var query = from s in context.ShoppingMalls
        join h in context.Houses
        on
        new { s.CouncilCode, s.PostCode }
        equals
         new { h.CouncilCode, h.PostCode }
        select s;

Answer 4

var query = from s in context.ShoppingMalls
join h in context.Houses
on
new {CouncilCode=s.CouncilCode, PostCode=s.PostCode }
equals
new {CouncilCode=h.District, PostCode=h.ZipCode }
select s;

这适用于任何类型的数据类型.