LINQ

            //INNER JOIN
            var query = (from u in db.UserBase
                         from g in db.UserGroup
                         where u.ID == g.PID
                         select u).ToList();
            //INNER JOIN
            var query = (from a in db.UserBase
                         join b in db.UserGroup on a.ID equals b.PID
                         select new Test { id = a.ID, pid = b.PID, name = a.LoginName, Group = b.GroupName }
                        ).ToList();

            //INNER JOIN
            var query = (from a in db.UserBase
                         join b in db.UserGroup on a.ID equals b.PID
                         join c in db.WorkGroup on b.GroupName equals c.GroupName
                         select new Test { id = a.ID, pid = b.PID, name = a.LoginName, Group = b.GroupName }
                         ).ToList();
            //CROSS JOIN
            var query = (from u in db.UserBase
                         from g in db.UserGroup
                         select u).ToList();

            //LEFT OUTER JOIN 
            var query = (from a in db.UserBase
                         join b in db.UserGroup on a.ID equals b.PID into order
                         from o in order.DefaultIfEmpty()
                         //where o != null
                         select new Test
                         {
                             id = a.ID,
                             Group = o == null ? "null" : o.GroupName,
                             name = a.LoginName,
                             pid = o == null ? 0 : o.PID
                         }
                       ).ToList();