Getting fortnight from timestamp in Postgres - postgresql

Question

I'm doing some cohort analysis and want to see for a group of customers in November, how many transact weekly, fortnightly, and monthly; and for how long
I have this for the week and month (weekly example):
WITH weekly_users AS (
SELECT user_fk
, DATE_TRUNC('week',created_at) AS week
, (DATE_PART('year', created_at) - 2016) * 52 + DATE_PART('week', created_at) - 45 AS weeks_between
FROM transactions
WHERE created_at >= '2016-11-01' AND created_at < '2017-12-01'
GROUP BY user_fk, week, weeks_between
),
t2 AS (
SELECT weekly_users.*
, COUNT(*) OVER (PARTITION BY user_fk
ORDER BY week ROWS BETWEEN UNBOUNDED PRECEDING
AND 1 PRECEDING) AS prev_rec_cnt
FROM weekly_users
)
SELECT week
, COUNT(*)
FROM t2
WHERE weeks_between = prev_rec_cnt
GROUP BY week
ORDER BY week;
But weekly is too little of an interval, and monthly too much. So I want fortnight. Has anyone done this before? From Googling it seems like a challenge
Thanks in advance

Answer 1

Just worked it out, this is how you'd do it:
WITH fortnightly_users AS (
SELECT user_fk
, EXTRACT(YEAR FROM created_at) * 100 + CEIL(EXTRACT(WEEK FROM created_at)/2) AS fortnight
, (EXTRACT(YEAR FROM created_at) - 2016) * 26 + CEIL(EXTRACT(WEEK FROM created_at)/2) - 23 AS fortnights_between
FROM transactions
WHERE created_at >= '2016-11-01' AND created_at < '2017-12-01'
GROUP BY user_fk, fortnight, fortnights_between
),
t2 AS (
SELECT fortnightly_users.*
, COUNT(*) OVER (PARTITION BY user_fk
ORDER BY fortnight ROWS BETWEEN UNBOUNDED PRECEDING
AND 1 PRECEDING) AS prev_rec_cnt
FROM fortnightly_users
)
SELECT fortnight
, COUNT(*)
FROM t2
WHERE fortnights_between = prev_rec_cnt
GROUP BY fortnight
ORDER BY fortnight;
So you get the week number, then divide by 2. Rounding up to avoid fractional numbers for fortnights