PostgreSQL GROUP BY: SELECT column on MAX of another WHERE a third column = x - sql

Question

Let's suppose we have two tables in PostgreSQL:
Table "citizens"
country_ref citizen_name entry_date
-----------------------------------------------------
0 peter 2013-01-14 21:00:00.000
1 fernando 2013-01-14 20:00:00.000
0 robert 2013-01-14 19:00:00.000
3 albert 2013-01-14 18:00:00.000
2 esther 2013-01-14 17:00:00.000
1 juan 2013-01-14 16:00:00.000
3 egbert 2013-01-14 15:00:00.000
1 francisco 2013-01-14 14:00:00.000
3 adolph 2013-01-14 13:00:00.000
2 emilie 2013-01-14 12:00:00.000
2 jacques 2013-01-14 11:00:00.000
0 david 2013-01-14 10:00:00.000
Table "countries"
country_id country_name country_group
-------------------------------------------
0 england 0
1 spain 0
2 france 1
3 germany 1
Now I want to obtain the last entered citizen on the "citizens" table for each country of a given country_group.
My best try so far is this query (Let's call it Query_1) :
SELECT country_ref, MAX(entry_date) FROM citizens
LEFT JOIN countries ON country_id = country_ref
WHERE country_group = 1 GROUP BY country_ref
Output:
country_ref max
---------------------------------
3 2013-01-14 18:00:00
2 2013-01-14 17:00:00
So then I could do:
SELECT citizen_name FROM citizens WHERE (country_ref, entry_date) IN (Query_1)
... which will give me the output I'm looking for: albert and esther.
But I'd prefer to achieve this in a single query. I wonder if it's possible?

Answer 1

This should be simplest and fastest:
SELECT DISTINCT ON (i.country_ref)
i.citizen_name
FROM citizens i
JOIN countries o ON o.country_id = i.country_ref
WHERE o.country_group = 1
ORDER BY i.country_ref, i.entry_date DESC
You can easily return more columns from both tables by simply adding them to the SELECT list.
SQL Fiddle.
Details, links and explanation in this related answer:
Select first row in each GROUP BY group?

Answer 2

SELECT citizen_name,
country_ref,
entry_date
from (
SELECT cit.citizen_name,
cit.country_ref,
MAX(cit.entry_date) over (partition by cit.country_ref) as max_date,
cit.entry_date
FROM citizens cit
LEFT JOIN countries cou ON cou.country_id = cit.country_ref
WHERE cou.country_group = 1
) t
where max_date = entry_date
SQLFiddle demo: http://www.sqlfiddle.com/#!12/50776/1

Answer 3

Why don't you simply:
SELECT citizen_name FROM citizens WHERE (country_ref, entry_date) IN (
SELECT country_ref, MAX(entry_date) FROM citizens
LEFT JOIN countries ON country_id = country_ref
WHERE country_group = 1 GROUP BY country_ref
)
It might not be the best plan, but it depends on many factors, and it is simple to write.