Count / sum values in subquery and order by it - sql

Question

I have tables like below:
user
id | status
1 | 0
gallery
id | status | create_by_user_id
1 | 0 | 1
2 | 0 | 1
3 | 0 | 1
media
id | status
1 | 0
2 | 0
3 | 0
gallery_media
fk gallery.id fk media.id
id | gallery_id | media_id | sequence
1 | 1 | 1 | 1
2 | 2 | 2 | 1
3 | 2 | 3 | 2
monitor_traffic
1:gallery 2:media
id | anonymous_id | user_id | endpoint_code | endpoint_id
1 | 1 | | 1 | 2 gallery.id 2
2 | 2 | | 1 | 2 gallery.id 2
3 | | 1 | 2 | 3 media.id 3 include in gallery.id 2
these means gallery.id 2 contain 3 rows
gallery_information
fk gallery.id
id | gallery_id
gallery includes media.
monitor_traffic.endpoint_code: 1 .. gallery; 2 .. media
If 1 then monitor_traffic.endpoint_id references gallery.id
monitor_traffic.user_id, monitor_traffic.anonymous_id integer or null
Objective
I want to output gallery rows sort by count each gallery rows in monitor_traffic, then count the gallery related media rows in monitor_traffic. Finally sum them.
The query I provide only counts media in monitor_traffic without summing them and also does not count gallery in monitor_traffic.
How to do this?
This is part of a function, input option then output build query, something like this. I hope to find a solution (maybe with a subquery) that does not require to change other parts of the query.
Query:
SELECT
g.*,
row_to_json(gi.*) as gallery_information
FROM gallery g
LEFT JOIN gallery_information gi ON gi.gallery_id = g.id
LEFT JOIN "user" u ON u.id = g.create_by_user_id
-- start
LEFT JOIN gallery_media gm ON gm.gallery_id = g.id
LEFT JOIN (
SELECT
endpoint_id,
COUNT(*) as mt_count
FROM monitor_traffic
WHERE endpoint_code = 2
GROUP BY endpoint_id
) mt ON mt.endpoint_id = m.id
-- end
ORDER BY mt.mt_count desc NULLS LAST;
sql fiddle

Answer 1

I suggest a CTE to count both types in one aggregation and join to it two times in the FROM clause:
WITH mt AS ( -- count once for both media and gallery
SELECT endpoint_code, endpoint_id, count(*) AS ct
FROM monitor_traffic
GROUP BY 1, 2
)
SELECT g.*, row_to_json(gi.*) AS gallery_information
FROM gallery g
LEFT JOIN mt ON mt.endpoint_id = g.id -- 1st join to mt
AND mt.endpoint_code = 1 -- gallery
LEFT JOIN (
SELECT gm.gallery_id, sum(ct) AS ct
FROM gallery_media gm
JOIN mt ON mt.endpoint_id = gm.media_id -- 2nd join to mt
AND mt.endpoint_code = 2 -- media
GROUP BY 1
) mmt ON mmt.gallery_id = g.id
LEFT JOIN gallery_information gi ON gi.gallery_id = g.id
ORDER BY mt.ct DESC NULLS LAST -- count of galleries
, mmt.ct DESC NULLS LAST; -- count of "gallery related media"
Or, to order by the sum of both counts:
...
ORDER BY COALESCE(mt.ct, 0) + COALESCE(mmt.ct, 0) DESC;
Aggregate first, then join. That prevents complications with "proxy-cross joins" that multiply rows:
Two SQL LEFT JOINS produce incorrect result
The LEFT JOIN to "user" seems to be dead freight. Remove it:
LEFT JOIN "user" u ON u.id = g.create_by_user_id
Don't use reserved words like "user" as identifier, even if that's allowed as long as you double-quote. Very error-prone.