Question

The following code generates dates and counts records by day.

SELECT ts, COUNT(DISTINCT(user_id)) FROM 
( SELECT current_date + s.ts FROM generate_series(-20,0,1) AS s(ts) )
AS series(ts)
LEFT JOIN messages
ON messages.created_at::date = ts
GROUP BY ts
ORDER BY ts

The output looks like:

2011-07-07   0
2011-07-08   0
2011-07-09   0
2011-07-10   0
2011-07-11   0
2011-07-12   94
2011-07-13   56
2011-07-14   35
2011-07-15   56
2011-07-16   0
2011-07-17   13

How would you modify it to group by 2 days, so that the results overlap? Instead of counting the distinct user_id’s for each day, it would count the distinct user_id’s for each 2 day period.

This is different from summing the counts of the 2 days, as the user_id should be counted only once for each 2 day period.

Working in PostgreSQL 8.3.

Thanks.

Answer 1

SELECT ts, COUNT(DISTINCT(user_id)) FROM 
( SELECT current_date + s.ts FROM generate_series(-20,0,1) AS s(ts) )
AS series(ts)
LEFT JOIN messages
ON messages.created_at::date between ts - 1 and ts -- JOIN on a range
GROUP BY ts
ORDER BY ts