GROUP BY column and clause in postgres - postgresql

Question

I would like group the columns of a table with by a column value as well as when another condition is met. For example, with the following table:
Events:
id session_id flags created_at ...
--------------------------------------------
1 100 OTHER ...
2 101 OTHER ...
3 101 NEW_SESSION ...
4 101 OTHER ...
5 101 NEW_SESSION ...
6 100 OTHER ...
7 102 OTHER ...
I want the following result:
session_id events_count first_event_id last_event_id
-------------------------------------------------------
100-0 2 1 6
101-0 1 2 2
101-1 2 3 4
101-2 1 5 5
102-0 1 7 7
The basic idea is that I want to extract sessions from events. They are grouped by session_id. I also want a new session whenever I have the flag NEW_SESSION.
The query is something like this:
SELECT ? as session_id
, count(id) as events_count
, MIN(id) as first_event_id
, MAX(id) last_event_id
GROUP BY session_id
-- , and whenever flags is NEW_SESSION
ORDER BY id
But I dont know how to express the group by condition properly. Any idea ?

Answer 1

Update 2
In the comments I've noticed that you want them unique. Then we can use a variable:
SET #inc := 0;
(
SELECT CONCAT(session_id, '-', !ABS(STRCMP(flags, 'NEW_SESSION'))) AS session_id
, COUNT(id) AS events_count
, MIN(id) AS first_event_id
, MAX(id) last_event_id
FROM events
WHERE flags != 'NEW_SESSION'
GROUP BY events.session_id, events.flags
ORDER BY events.id
) UNION (
SELECT CONCAT(session_id, '-', #inc := #inc + 1) AS session_id
, COUNT(id) AS events_count
, MIN(id) AS first_event_id
, MAX(id) last_event_id
FROM events
WHERE flags = 'NEW_SESSION'
GROUP by events.id
ORDER BY events.id
);
Update
The following prevents grouping for the NEW_SESSION rows:
(
SELECT CONCAT(session_id, '-', !ABS(STRCMP(flags, 'NEW_SESSION'))) AS session_id
, COUNT(id) AS events_count
, MIN(id) AS first_event_id
, MAX(id) last_event_id
FROM events
WHERE flags != 'NEW_SESSION'
GROUP BY events.session_id, events.flags
ORDER BY events.id
) UNION (
SELECT CONCAT(session_id, '-1') AS session_id
, COUNT(id) AS events_count
, MIN(id) AS first_event_id
, MAX(id) last_event_id
FROM events
WHERE flags = 'NEW_SESSION'
GROUP BY id
ORDER BY events.id
);
Original answer
As far as I understand, you are trying to group events by the session IDs and
"whether it's a NEW_SESSION" flag. If it's so, then I'd express it as follows:
SELECT CONCAT(session_id, '-', !ABS(STRCMP(flags, 'NEW_SESSION'))) AS session_id
, COUNT(id) AS events_count
, MIN(id) AS first_event_id
, MAX(id) last_event_id
FROM events
GROUP BY events.session_id, events.flags
ORDER BY events.id;