Postgres query for calendar - sql

Question

I am trying to write a query to retrieve data from an events query for a simple calendar app.
The table structure is as followed:
table name: events
Column | Type
---------+-----------
id | integer
start | timestamp
end | timestamp
the data inside of the table
id| start | end
--+---------------------+--------------------
1 | 2017-09-01 12:00:00 | 2017-09-01 12:00:00
2 | 2017-09-03 10:00:00 | 2017-09-03 12:00:00
3 | 2017-09-08 12:00:00 | 2017-09-11 12:00:00
4 | 2017-09-11 12:00:00 | 2017-09-11 12:00:00
the expected result is
date | event.id
-----------+---------
2017-09-01 | 1
2017-09-03 | 2
2017-09-08 | 3
2017-09-09 | 3
2017-09-10 | 3
2017-09-11 | 3
2017-09-11 | 4
As you can see, only days with an event (not just start and end, but also the days in between) is retrieved, days without an event are not retrieved at all.
In the second step I would like to be able to limit the amount of distinct days, e.g. "get 4 days with events" what might be more than 4 rows.
Right now I am able to retrieve the events based on start date only using the following query:
SELECT start::date, id FROM events WHERE events.start::date >= '2017-09-01' LIMIT 3
Thinks I already though about are DENSE_RANK and generate_series, but up to now I didn't find a way to fill the gaps between start and end, but not on days where there are no data.
So in short:
What I want to get is: get the next X days where there is an event. A date with an event is a day where start <= date >= end
Any ideas ?
Edit
Thanks to Tim I have now the following query (modified to use generate_series instead of a table and added a limit using dense_rank):
select date, id FROM (
SELECT
DENSE_RANK() OVER (ORDER BY t1.date) as rank,
t1.date,
events.id
FROM
generate_series([DATE]::date, [DATE]::date + interval '365 day', '1 day') as t1
INNER JOIN
events
ON t1.date BETWEEN events.start::date AND events."end"::date
) as t
WHERE rank <= [LIMIT]
This is working really good, even though I am not 100% sure about the performance hit with this kind of limit

Answer 1

I think you really need a calendar table here to cover the full range of dates in which your data may appear. In the first CTE below, I generate a table covering the month of September 2017. Then all we need to do is inner join this calendar table with the events table on the criteria of a given day appearing within a given range.
WITH cte AS (
SELECT CAST('2017-09-01' AS DATE) + (n || ' day')::INTERVAL AS date
FROM generate_series(0, 29) n
)
SELECT
t1.date,
t2.id
FROM cte t1
INNER JOIN events t2
ON t1.date BETWEEN CAST(t2.start AS DATE) AND CAST(t2.end AS DATE);
Output:
date id
1 01.09.2017 00:00:00 1
2 03.09.2017 00:00:00 2
3 08.09.2017 00:00:00 3
4 09.09.2017 00:00:00 3
5 10.09.2017 00:00:00 3
6 11.09.2017 00:00:00 3
7 11.09.2017 00:00:00 4
Demo here:
Rextester