Getting the days between two dates for multiple IDs - sql

Question

This is a follow up question from How to get a list of months between 2 given dates using a query?
really. (I suspect it's because I don't quite understand the logic behind connect by level clauses !)
What I have is a list of data like so
ID | START_DATE | END_DATE
1 | 01-JAN-2018 | 20-JAN-2018
2 | 13-FEB-2018 | 20-MAR-2018
3 | 01-MAR-2018 | 07-MAR-2018
and what I want to try and get is a list with all the days between the start and end date for each ID.
So for example I want a list which gives
ID | DATE
1 | 01-JAN-2018
1 | 02-JAN-2018
1 | 03-JAN-2018
...
1 | 19-JAN-2018
1 | 20_JAN-2018
2 | 13-FEB-2018
2 | 14-FEB-2018
2 | 15-FEB-2018
...
etc.
What I've tried to do is adapt one of the answers from the above link as follows
select id
, trunc(start_date+((level-1)),'DD')
from (
select id
, start_date
, end_date
from blah
)
connect by level <= ((trunc(end_date,'DD')-trunc(start_date,'DD'))) + 1
which gives me what I want but then a whole host of duplicate dates as if it's like a cartesian join. Is there something simple I need to add to fix this?

Answer 1

I like recursive CTEs:
with cte as (
select id, start_dte as dte, end_dte
from blah
union all
select id, dte + 1, end_dte
from cte
where dte < end_dte
)
select *
from cte
order by id, dte;
This is ANSI standard syntax and works in several other databases.

Answer 2

The hierarchical query you were trying to do needs to include id = prior id in the connect-by clause, but as that causes loops with multiple source rows you also need to include a call to a non-deterministic function, such as dbms_random.value:
select id, start_date + level - 1 as day
from blah
connect by level <= end_date - start_date + 1
and prior id = id
and prior dbms_random.value is not null
With your sample data in a CTE, that gets 63 rows back:
with blah (ID, START_DATE, END_DATE) as (
select 1, date '2018-01-01', date '2018-01-20' from dual
union all select 2, date '2018-02-13', date '2018-03-20' from dual
union all select 3, date '2018-03-01', date '2018-03-07' from dual
)
select id, start_date + level - 1 as day
from blah
connect by level <= end_date - start_date + 1
and prior id = id
and prior dbms_random.value is not null;
ID DAY
---------- ----------
1 2018-01-01
1 2018-01-02
1 2018-01-03
...
1 2018-01-19
1 2018-01-20
2 2018-02-13
2 2018-02-14
...
3 2018-03-05
3 2018-03-06
3 2018-03-07
You don't need to trunc() the dates unless they have non-midnight times, which seems unlikely in this case, and even then it might not be necessary if only the end-date has a later time (like 23:59:59).
A recursive CTE is more intuitive in many ways though, at least once you understand the basic idea of them; so I'd probably use Gordon's approach too. There can be differences in performance and whether they work at all for large amounts of data (or generated rows), but for a lot of data it's worth comparing different approaches to find the most suitable/performant anyway.