How to group by more than one row value? - sql

Question

I am working with POSTGRESQL and I can't find out how to solve a problem. I have a model called Foobar. Some of its attributes are:
FOOBAR
check_in:datetime
qr_code:string
city_id:integer
In this table there is a lot of redundancy (qr_code is not unique) but that is not my problem right now. What I am trying to get are the foobars that have same qr_code and have been in a well known group of cities, that have checked in at different moments.
I got this by querying:
SELECT * FROM foobar AS a
WHERE a.city_id = 1
AND EXISTS (
SELECT * FROM foobar AS b
WHERE a.check_in < b.check_in
AND a.qr_code = b.qr_code
AND b.city_id = 2
AND EXISTS (
SELECT * FROM foobar as c
WHERE b.check_in < c.check_in
AND c.qr_code = b.qr_code
AND c.city_id = 3
AND EXISTS(...)
)
)
where '...' represents more queries to get more persons with the same qr_code, different check_in date and those well known cities.
My problem is that I want to group this by qr_code, and I want to show the check_in fields of each qr_code like this:
2015-11-11 14:14:14 => [2015-11-11 14:14:14, 2015-11-11 16:16:16, 2015-11-11 17:18:20] (this for each different qr_code)
where the data at the left is the 'smaller' date for that qr_code, and the right part are all the other dates for that qr_code, including the first one.
Is this possible to do with a sql query only? I am asking this because I am actually doing this app with rails, and I know that I can make a different approach with array methods of ruby (a solution with this would be well received too)

Answer 1

You could solve that with a recursive CTE - if I interpret your question correctly:
Assuming you have a given list of cities that must be visited in order by the same qr_code. Your text doesn't say so, but your query indicates as much.
WITH RECURSIVE
c AS (SELECT '{1,2,3}'::int[] AS cities) -- your list of city_id's here
, route AS (
SELECT f.check_in, f.qr_code, 2 AS idx
FROM foobar f
JOIN c ON f.city_id = c.cities[1]
UNION ALL
SELECT f.check_in, f.qr_code, r.idx + 1
FROM route r
JOIN foobar f USING (qr_code)
JOIN c ON f.city_id = c.cities[r.idx]
WHERE r.check_in < f.check_in
)
SELECT qr_code, array_agg(check_in) AS check_in_list
FROM (
SELECT *
FROM route
ORDER BY qr_code, idx -- or check_in
) sub
HAVING count(*) = (SELECT array_length(cities) FROM c);
GROUP BY 1;
Provide the list as array in the first (non-recursive) CTE c.
In the recursive part start with any rows in the first city and travel along your array until the last element.
In the final SELECT aggregate your check_in column in order. Only return qr_code that have visited all cities of the array.
Similar:
Recursive query used for transitive closure