Linq中使用Left Join

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
                    };