var query=from u in user
          join up in userdepartment on u.user_userdepartmentid equals up.userdepartment_id
          join ug in usergroup on ug.usergroup_userid equals u.user_id
          join g in group on g.group_id equals ug.usergroup_groupid
          where u.suer_name=="andy" || up.userdepartment=="123" || g.group_name=="213456"
          select new {u,up,ug,g};

------解决方案--------------------
建好主外键关系后，直接 var data=from a in context.as
                            from b  in context.bs
                             select a;