grouping by column but getting multiple results for each - sql

Question

I am trying to calculate the median response time for conversations on each date for the last X days.
I use the following query below, but for some reason, it will generate multiple rows with the same date.
with grouping as (
SELECT a.id, d.date, extract(epoch from (first_response_at - started_at)) as response_time
FROM (
select to_char(date_trunc('day', (current_date - offs)), 'YYYY-MM-DD') AS date
FROM generate_series(0, 2) AS offs
) d
LEFT OUTER JOIN apps a on true
LEFT OUTER JOIN conversations c ON (d.date=to_char(date_trunc('day'::varchar, c.started_at), 'YYYY-MM-DD')) and a.id = c.app_id
and c.app_id = a.id and c.first_response_at > (current_date - (2 || ' days')::interval)::date
)
select
*
from grouping
where grouping.id = 'ASnYW1-RgCl0I'
Any ideas?

Answer 1

First a number of issues with your query, assuming there aren't any parts you haven't shown us:
You don't need a CTE for this query.
From table apps you only use column id whose value is the same as c.app_id. You can remove the table apps and select c.app_id for the same result.
When you use to_char() you do not first have to date_trunc() to a date, the to_char() function handles that.
generate_series() also works with timestamps. Just enter day values with an interval and cast the end result to date before using it.
So, removing all the flotsam we end up with this which does exactly the same as the query in your question but now we can at least see what is going on.
SELECT c.app_id, to_date(d.date, 'YYYY-MM-DD') AS date,
extract(epoch from (first_response_at - started_at)) AS response_time
FROM generate_series(CURRENT_DATE - 2, CURRENT_DATE, interval '1 day') d(date)
LEFT JOIN conversations c ON d.date::date = c.started_at::date
AND c.app_id = 'ASnYW1-RgCl0I'
AND c.first_response_at > CURRENT_DATE - 2;
You don't calculate the median response time anywhere, so that is a big problem you need to solve. This only requires data from table conversations and would look somewhat like this to calculate the median response time for the past 2 days:
SELECT app_id, started_at::date AS start_date,
percentile_disc(0.5) WITHIN GROUP (ORDER BY first_response_at - started_at) AS median_response
FROM conversations
WHERE app_id = 'ASnYW1-RgCl0I'
AND first_response_at > CURRENT_DATE - 2
GROUP BY 2;
When we fold the two queries, and put the parameters handily in a single place, this is the final result:
SELECT p.id, to_date(d.date, 'YYYY-MM-DD') AS date,
extract(epoch from (c.median_response)) AS response_time
FROM (VALUES ('ASnYW1-RgCl0I', 2)) p(id, days)
JOIN generate_series(CURRENT_DATE - p.days, CURRENT_DATE, interval '1 day') d(date) ON true
LEFT JOIN LATERAL (
SELECT started_at::date AS start_date,
percentile_disc(0.5) WITHIN GROUP (ORDER BY first_response_at - started_at) AS median_response
FROM conversations
WHERE app_id = p.id
AND first_response_at > CURRENT_DATE - p.days
GROUP BY 2) c ON d.date::date = c.start_date;
If you want to change the id of the app or the number of days to look back, you only have to change the VALUES clause accordingly. You can also wrap the whole thing in a SQL function and convert the VALUES clause into two parameters.