Using generate_serires with partition by generate triple duplicate row - sql

Question

I have a table which look like this:
dt type
-----------------------------
2019-07-01 10:00:00 A
2019-07-01 10:15:00 A
2019-07-01 11:00:00 A
2019-07-01 08:30:00 B
2019-07-01 08:45:00 B
2019-07-01 09:30:00 B
Each type has it own dt value but each type should have a consecutive 15 minute range dt. But some row are missing. So, I used generate_strings() to add date and partition by to do it based on each type column by using this:
SELECT
generate_series(min(dt) over (partition by type),
max(dt) over (partition by type), interval '15 minute')
, type
FROM t
which I generate datetime in dt column based on in min to max dt with a range of 15 minutes.
This is what I expect to get:
dt type
-----------------------------
2019-07-01 10:00:00 A
2019-07-01 10:15:00 A
2019-07-01 10:30:00 A
2019-07-01 10:45:00 A
2019-07-01 11:00:00 A
2019-07-01 08:30:00 B
2019-07-01 08:45:00 B
2019-07-01 09:00:00 B
2019-07-01 09:15:00 B
2019-07-01 09:30:00 B
But what I got as a result is like the expected one but it return triple for each type and datetime.
E.g.
dt type
-----------------------------
2019-07-01 10:00:00 A
2019-07-01 10:15:00 A
2019-07-01 10:30:00 A
2019-07-01 10:45:00 A
2019-07-01 11:00:00 A
2019-07-01 10:00:00 A
2019-07-01 10:15:00 A
2019-07-01 10:30:00 A
2019-07-01 10:45:00 A
2019-07-01 11:00:00 A
2019-07-01 10:00:00 A
2019-07-01 10:15:00 A
2019-07-01 10:30:00 A
2019-07-01 10:45:00 A
2019-07-01 11:00:00 A
2019-07-01 08:30:00 B
. . .
This also happened to type B as well.
So, from my query, what do I need to change to get the expected result?

Answer 1

You just want to run generate_series() over the aggregation:
SELECT type, generate_series(min_dt, max_dt, interval '15 minute')
FROM (SELECT type, MIN(dt) as min_dt, MAX(dt) as max_dt
FROM t
GROUP BY type
) t;
The window functions start by adding the min and max value to each row. Then each row gets its own series.