linq的左连接右连接内连接用法

1、左连接：

var LeftJoin = from e in ListOfEmployees
join d in ListOfDepartment
on e.DeptID equals d.ID into JoinedEmpDept
from d in JoinedEmpDept.DefaultIfEmpty()
select
new

{
EmployeeName = e.Name,
DepartmentName = d!= null ? d.Name :
null

};

2、右连接：

var RightJoin = from d in
ListOfDepartment
join e in ListOfEmployees
on d.ID equals e.DeptID into joinDeptEmp
from e in joinDeptEmp.DefaultIfEmpty()
select
new

{
EmployeeName = e!= null ? e.Name : null,
DepartmentName = d.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,

                 };

三个表链表查询

var list = (
    from u in dc.userinfos
        join j in dc.jobs on u.job equals j.jid into j_join
    from x in j_join.DefaultIfEmpty()
        join c in dc.cities on u.city equals c.cid into c_join
    from v in c_join.DefaultIfEmpty()
    select new
    {
        id = u.id,
        name = u.name,
        jname = x.jname,
        cname = v.cname,
        /*u1=u,x1=x,v1=v*/
        //不要用对象的方式 因为对象可能为null那么对象.属性就会抛异常
    }
    ).ToList();