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
};
如果想实现右连接，就把两个表换一下位置。
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, //重点
DepartmentName = dept.Name
};
左链接或右链接，使用DefaultIfEmpty()语法，采用些语法前需要用到into语法
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,
};