在Postgresql中选择多个具有别名的列并按别名分组

Question

I'm having a problem with this query. I want to select the ID field and then a date timestamp field has an alias but group by the alias. I can't do it without including the ID field but then then it doesn't keep the uniqueness the group by provides. Here is my query which doesn't work. I get an error about needing to include the ID field.

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

UPDATE: After using a function as in below it worked as needed.

   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

Answer 1

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