Find last occurring value within record in PostgreSQL - postgresql

Question

I'm not new to SQL, but I am new to PostgreSQL and am really struggling to adapt my current knowledge in a different environment.
I am trying to create a variable that captures whether or not someone stays active, skips, or churns within a 0/1 time series variable. For example, in the data below, my dataset would include the variables id,time, and voted, and I would create the variable "skipped":
id time voted skipped
1 1 1 active
1 2 0 skipped
1 3 1 active
2 1 1 active
2 2 0 churned
2 3 0 churned
3 1 1 active
3 2 1 active
3 3 0 churned
The rule for coding "skipped" is pretty simple: If 1 is the last record, the person is "active" and any zeroes count as "skipped", but if 0 is the last record, the person is "churned".
The record with id = 1 is a skip because id is non-zero at time 3 after being 0 at time 2. The other two cases, 0 is the final value so they are "churned". Can anyone help? I've been noodling on it all day, and am hitting a wall.

Answer 1

This isn't particularly elegant, but it should meet your needs:
with votes as (
select
id, time, voted,
max(time) over (partition by id) as max_time
from voter_data
)
select
v1.id, v1.time, v1.voted,
case
when v1.voted = 1 then 'active'
when v2.voted = 1 then 'skipped'
else 'churned'
end as skipped
from
votes v1
join votes v2 on
v1.id = v2.id and
v1.max_time = v2.time
In a nutshell, we first figure out which is the last record for each voter id, and then we do a self-join on the resulting table to isolate only that last id.
There is a chance this could produce multiple results -- if it's possible to have the same ID vote twice at the same time. If that's the case, you want row_number() instead of max().
Results on your data:
1 1 1 'active'
1 2 0 'skipped'
1 3 1 'active'
2 1 1 'active'
2 2 0 'churned'
2 3 0 'churned'
3 1 1 'active'
3 2 1 'active'
3 3 0 'churned'

Answer 2

Window functions can help for readability when working with self-referential joins.
WITH
add_last_voted_status AS (
SELECT
*
, LAST_VALUE(voted) OVER (
PARTITION BY id
ORDER BY time
) AS last_voted_status
FROM table
)
SELECT
id
, time
, voted
, CASE
WHEN last_voted_status = 0
THEN 'churned'
WHEN last_voted_status = 1 AND voted = 1
THEN 'active'
WHEN last_voted_status = 1 AND voted = 0
THEN 'skipped'
ELSE '?'
END AS skipped
FROM add_last_voted_status