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