Linq中使用Left Join rught join

　　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