postgres 列别名问题

As a newbie to Postgresql (I'm moving over because I'm moving my site to heroku who only support it, I'm having to refactor some of my queries and code. Here's a problem that I can't quite understand the problem with:

PGError: ERROR:  column "l_user_id" does not exist
LINE 1: ...t_id where l.user_id = 8 order by l2.geopoint_id, l_user_id ...
                                                             ^

...查询:

   select distinct 
          l2.*, 
          l.user_id as l_user_id, 
          l.geopoint_id as l_geopoint_id 
     from locations l 
left join locations l2 on l.geopoint_id = l2.geopoint_id 
    where l.user_id = 8 
 order by l2.geopoint_id, l_user_id = l2.user_id desc

子句l.user_id as l_user_id, l.geopoint_id as l_geopoint_id"被添加，因为显然 postgres 不喜欢没有选择字段的 order 子句.但是我现在得到的错误看起来好像我也没有走样.任何有 postgres 经验的人都看到了这个问题吗?

   select distinct 
          l2.*, 
          l.user_id as l_user_id, 
          l.geopoint_id as l_geopoint_id 
     from locations l 
left join locations l2 on l.geopoint_id = l2.geopoint_id 
    where l.user_id = 8 
 order by l2.geopoint_id, l.user_id = l2.user_id desc;