Linq中使用Left Join 和 Right Join

原文地址：http://www.cnblogs.com/xinjian/archive/2010/11/17/1879959.html

准备一些测试数据，如下：

use Test Create table Student( ID int identity(1,1) primary key, [Name] nvarchar(50) not null )
Create Table Book( ID int identity(1,1) primary key, [Name] nvarchar(50)not null, StudentID int not null )
insert into Student values('张三') insert into Student values('李四') insert into Student values('王五') select * from student
--张三借的书 insert into Book values('红楼',1) insert into Book values('大话红楼',1)
--李四借的书 insert into Book values('三国',2)
--王五没借书 --一本错误的记录 insert into Book values('错误时怎样练成的',111)
--左连接 select s.name,b.name from student as s left join Book as b on s.id=b.studentid
--右连接 select s.name,b.name from student as s right join Book as b on s.id=b.studentid

要用Linq实现左连接，写法如下

DataClasses1DataContext db = new DataClasses1DataContext(); var leftJoinSql = from student in db.Student join book in db.Book on student.ID equals book.StudentID into temp from tt in temp.DefaultIfEmpty() select new { sname= student.Name, bname = tt==null?"":tt.Name//这里主要第二个集合有可能为空。需要判断 };

用Linq实现右连接，写法如下

DataClasses1DataContext db=new DataClasses1DataContext(); var rightJoinSql = from book in db.Book join stu in db.Student on book.StudentID equals stu.ID into joinTemp from tmp in joinTemp.DefaultIfEmpty() select new { sname=tmp==null?"":tmp.Name, bname=book.Name
};

参考资料：http://developer.51cto.com/art/200909/152189.htm

http://hi.baidu.com/thinsoft/blog/item/83fb1e9089cc7186a877a4b1.html

http://apps.hi.baidu.com/share/detail/12540006

http://www.winu.cn/space-14160-do-blog-id-25172.html

转载于:https://www.cnblogs.com/xbzhu/p/7373155.html

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