[转]Linq中使用Left Join

本文转自：http://www.cnblogs.com/xinjian/archive/2010/11/17/1879959.html

use Test
Create table Student(
ID int identity(,) primary key,
[Name] nvarchar() not null
)

Create Table Book(
ID int identity(,) primary key,
[Name] nvarchar()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',)
insert into Book values('大话红楼',)

--李四借的书
insert into Book values('三国',)

--王五没借书

--一本错误的记录
insert into Book values('错误时怎样练成的',)

--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

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要用Linq实现左连接，写法如下

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            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
                              };

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用Linq实现右连接，写法如下

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            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

                               };

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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