Linq实现左连接、右连接

--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

};

1、左连接：

var LeftJoin = from emp in ListOfEmployees
join dept in ListOfDepartment
on emp.DeptID equals dept.ID into JoinedEmpDept
from dept in JoinedEmpDept.DefaultIfEmpty()
select new
{
EmployeeName = emp.Name,
DepartmentName = dept != null ? dept.Name : null
};

2、右连接：

var RightJoin = from dept in ListOfDepartment
join employee in ListOfEmployees
on dept.ID equals employee.DeptID into joinDeptEmp
from employee in joinDeptEmp.DefaultIfEmpty()
select new
{
EmployeeName = employee != null ? employee.Name : null,
DepartmentName = dept.Name
};

3、内连接：

var query = from t in entitiy.TB_GCGL_ADA_USER
join p in entitiy.TB_GCGL_ZY_ZYK
on t.ETPRS_CODE equals p.ETPRS_CODE

                 select new TB_USER_ZYK
                 {
                    USER_ID = t.USER_ID,
                    USER_NAME = t.USER_NAME,
                    USER_PASSWORD = t.USER_PASSWORD,

};