linq 左连接 右连接 内连接

left join （左连接）：返回包括左表中的所有记录和右表中连接字段相等的记录。
right join （右连接）：返回包括右表中的所有记录和左表中连接字段相等的记录。
inner join （等值连接或者叫内连接）：只返回两个表中连接字段相等的行。

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,

                 };