在Postgresql中选择多个别名为且按别名分组的列

我对这个查询有问题。我想选择ID字段，然后日期时间戳字段有一个别名，但按别名分组。如果不包含ID字段，我就无法做到这一点，但这样它就无法保持组所提供的唯一性。这是我的查询，它不起作用。我得到一个关于需要包含ID字段的错误

SELECT id, to_timestamp(start_date)::date as sdate FROM events WHERE end_date > 1349306845 GROUP BY sdate ORDER BY sdate ASC LIMIT 20

更新： 在使用下面的函数后，它可以根据需要工作

   SELECT max(id) as id, to_timestamp(start_date)::date as sdate FROM events WHERE end_date > 1349306845 GROUP BY sdate ORDER BY sdate ASC LIMIT 20

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没关系，我只是想出来了，而且似乎很管用。

SELECT  id, to_timestamp(start_date)::date as sdate 
FROM events
WHERE start_date IN 
   (
     SELECT start_date as sdat 
     FROM events 
     WHERE end_date > 1349306845 
     GROUP BY sdat 
     ORDER BY sdat 
     ASC LIMIT 20 
   )

SELECT  id, to_timestamp(start_date)::date as sdate 
FROM events
WHERE start_date IN 
   (
     SELECT start_date as sdat 
     FROM events 
     WHERE end_date > 1349306845 
     GROUP BY sdat 
   )
ORDER BY sdate 
ASC LIMIT 20