Linq中使用Left Join 和 Right Join

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

准備一些測試數據，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('張三')
insert into Student values('李四')
insert into Student values('王五')
select * from student

--張三借的書
insert into Book values('紅樓',1)
insert into Book values('大話紅樓',1)

--李四借的書
insert into Book values('三國',2)

--王五沒借書

--一本錯誤的記錄
insert into Book values('錯誤時怎樣練成的',111)

--左連接
select s.name,b.name from student as s
left join Book as b on s.id=b.studentid

--右連接
select s.name,b.name from student as s
right join Book as b on s.id=b.studentid

 

要用Linq實現左連接，寫法如下

 

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//這里主要第二個集合有可能為空。需要判斷
                              };

 

用Linq實現右連接，寫法如下

 

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

                               };

 

參考資料：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