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,          
};

多张表内联

var rec=from a in 表A

join b in 表B on a.字段 equals b.字段

join c in  表C on b.字段 equals c.字段 
select new item
{
   item1=a.想要的字段,
item2=b.想要的字段,
item3=c.想要的字段,
}

LINQ的左连接、右连接、内连接