How to find the number of purchases over time intervals SQL - sql

Question

I'm using Redshift (Postgres), and Pandas to do my work. I'm trying to get the number of user actions, lets say purchases to make it easier to understand. I have a table, purchases that holds the following data:
user_id, timestamp , price
1, , 2015-02-01, 200
1, , 2015-02-02, 50
1, , 2015-02-10, 75
ultimately I would like the number of purchases over a certain timestamp. Such as
userid, 28-14_days, 14-7_days, 7
Here is what I have so far, I'm aware I don't have an upper limit on the dates:
SELECT DISTINCT x_days.user_id, SUM(x_days.purchases) AS x_num, SUM(y_days.purchases) AS y_num,
x_days.x_date, y_days.y_date
FROM
(
SELECT purchases.user_id, COUNT(purchases.user_id) as purchases,
DATE(purchases.timestamp) as x_date
FROM purchases
WHERE purchases.timestamp > (current_date - INTERVAL '%(x_days_ago)s day') AND
purchases.max_value > 200
GROUP BY DATE(purchases.timestamp), purchases.user_id
) AS x_days
JOIN
(
SELECT purchases.user_id, COUNT(purchases.user_id) as purchases,
DATE(purchases.timestamp) as y_date
FROM purchases
WHERE purchases.timestamp > (current_date - INTERVAL '%(y_days_ago)s day') AND
purchases.max_value > 200
GROUP BY DATE(purchases.timestamp), purchases.user_id) AS y_days
ON
x_days.user_id = y_days.user_id
GROUP BY
x_days.user_id, x_days.x_date, y_days.y_date
params={'x_days_ago':x_days_ago, 'y_days_ago':y_days_ago}
where these are set in python/pandas
x_days_ago = 14
y_days_ago = 7
But this didn't work out exactly as planned:
user_id x_num y_num x_date y_date
0 5451772 1 1 2015-02-10 2015-02-10
1 5026678 1 1 2015-02-09 2015-02-09
2 6337993 2 1 2015-02-14 2015-02-13
3 6204432 1 3 2015-02-10 2015-02-11
4 3417539 1 1 2015-02-11 2015-02-11
Even though I don't have an upper date to look between (so x is effectively searching from 14 days to now and y is 7 days to now, meaning overlap), in some cases y is higher.
Can anyone help me either fix this or give me a better way?
Thanks!

Answer 1

It might not be the most efficient answer, but you can generate each sum with a sub-select:
WITH
summed AS (
SELECT user_id, day, COUNT(1) AS purchases
FROM (SELECT user_id, DATE(timestamp) AS day FROM purchases) AS _
GROUP BY user_id, day
),
users AS (SELECT DISTINCT user_id FROM purchases)
SELECT user_id,
(SELECT SUM(purchases) FROM summed
WHERE summed.user_id = users.user_id
AND day >= DATE(NOW() - interval ' 7 days')) AS days_7,
(SELECT SUM(purchases) FROM summed
WHERE summed.user_id = users.user_id
AND day >= DATE(NOW() - interval '14 days')) AS days_14
FROM users;
(This was tested in Postgres, not in Redshift; but the Redshift documentation suggests that both WITH and DISTINCT are supported.) I would have liked to do this with a window, to obtain rolling sums; but it's a little onerous without generate_series().