Calculate 7 Day Retention with SQL - sql

Question

Given the following tables,
users page_views
+-----------------+-----------+ +----------+-----------+
| user_id |varchar| <----+ | pv_id | varchar |
| reg_ts |timestamp| | pv_ts | timestamp |
| reg_device |varchar| +----> | user_id | varchar |
| mktg_channel |varchar| | url | varchar |
+-----------------+-----------+ | device | varchar |
+----------+-----------+
Table "users" has one row per registered user.
Table "page_views" has one row per page view event.
What % of users who first visit on a given day came back again 1 week later?
I'm currently using SQLlite and created a sample database but my output is off...
Below is what I have so far:
-- day 1 active users
SELECT *
FROM page_views
LEFT JOIN page_views AS future_page_views
ON page_views.user_id = future_page_views.user_id
AND page_views.pv_ts = future_page_views.pv_ts - datetime(future_page_views.pv_ts, '+7 day')
-- day 7 retained users
SELECT
future_page_views.pv_ts,
COUNT(DISTINCT page_views.user_id) as active_users,
COUNT(DISTINCT future_page_views.user_id) as retained_users,
CAST(COUNT(DISTINCT future_page_views.user_id) / COUNT(DISTINCT page_views.user_id) AS float) retention
FROM page_views
LEFT JOIN page_views as future_page_views
ON page_views.user_id = future_page_views.user_id
AND page_views.pv_ts = future_page_views.pv_ts - datetime(page_views.pv_ts, '+7 day')
GROUP BY 1
Not sure if I should use Strftime function (DATEDIFF) in this instance to capture the 7 day. Open to any suggestions and feedback, thanks in advance.
EDIT**
Sample data below, based on the below data set,
I expect only user_id (8) to show up as 7 day retained (first day 2020-01-02) (last day 2020-01-09)
Desired Output:
User_ID
p.pv_ts as First_Day
f.pv_ts as Last_Day
Retention Days (i.e 1,2,3,4,5 days...)
% of users who visited and came back on day 7

Answer 1

You can look at just the first two page visits and then aggregate. This gives
select user_id, min(pv_ts) as first_ts,
nullif(max(pv_ts), min(pv_ts)) as second_ts
from (select pv.*,
row_number() over (partition by user_id order by pv_ts) as seqnum
from page_views pv
) pv
where seqnum <= 2
group by user_id;
Then to get the totals:
select count(*),
sum(case when second_ts < datetime(first_ts, '+7day') then 1 else 0 end)
from (select user_id, min(pv_ts) as first_ts,
nullif(max(pv_ts), min(pv_ts)) as second_ts
from (select pv.*,
row_number() over (partition by user_id order by pv_ts) as seqnum
from page_views pv
) pv
where seqnum <= 2
group by user_id
) u;