Month counts between dates - sql

Question

I have the below table. I need to count how many ids were active in a given month. So thinking I'll need to create a row for each id that was active during that month so that id can be counted each month. A row should be generated for a term_dt during that month.
active_dt term_dt id
1/1/2018 101
1/1/2018 5/15/2018 102
3/1/2018 6/1/2018 103
1/1/2018 4/25/18 104

Answer 1

Apparently this is a "count number of overlapping intervals" problem. The algorithm goes like this:
Create a sorted list of all start and end points
Calculate a running sum over this list, add one when you encounter a start and subtract one when you encounter an end
If two points are same then perform subtractions first
You will end up with list of all points where the sum changed
Here is a rough outline of the query. It is for SQL Server but could be ported to any RDBMS that supports window functions:
WITH cte1(date, val) AS (
SELECT active_dt, 1 FROM #t AS t
UNION ALL
SELECT COALESCE(term_dt, '2099-01-01'), -1 FROM #t AS t
-- if end date is null then assume the row is valid indefinitely
), cte2 AS (
SELECT date, SUM(val) OVER(ORDER BY date, val) AS rs
FROM cte1
)
SELECT YEAR(date) AS YY, MONTH(date) AS MM, MAX(rs) AS MaxActiveThisYearMonth
FROM cte2
GROUP BY YEAR(date), MONTH(date)
DB Fiddle

Answer 2

I was toying with a simpler query, that seemed to do the trick, for Oracle:
with candidates (month_start) as (
select to_date ('2018-' || column_value || '-01','YYYY-MM-DD')
from
table
(sys.odcivarchar2list('01','02','03','04','05',
'06','07','08','09','10','11','12'))
), sample_data (active_dt, term_dt, id) as (
select to_date('01/01/2018', 'MM/DD/YYYY'), null, 101 from dual
union select to_date('01/01/2018', 'MM/DD/YYYY'),
to_date('05/15/2018', 'MM/DD/YYYY'), 102 from dual
union select to_date('03/01/2018', 'MM/DD/YYYY'),
to_date('06/01/2018', 'MM/DD/YYYY'), 103 from dual
union select to_date('01/01/2018', 'MM/DD/YYYY'),
to_date('04/25/2018', 'MM/DD/YYYY'), 104 from dual
)
select c.month_start, count(1)
from candidates c
join sample_data d
on c.month_start between d.active_dt and nvl(d.term_dt,current_date)
group by c.month_start
order by c.month_start

Answer 3

An alternative solution would be to use a hierarchical query, e.g.:
WITH your_table AS (SELECT to_date('01/01/2018', 'dd/mm/yyyy') active_dt, NULL term_dt, 101 ID FROM dual UNION ALL
SELECT to_date('01/01/2018', 'dd/mm/yyyy') active_dt, to_date('15/05/2018', 'dd/mm/yyyy') term_dt, 102 ID FROM dual UNION ALL
SELECT to_date('01/03/2018', 'dd/mm/yyyy') active_dt, to_date('01/06/2018', 'dd/mm/yyyy') term_dt, 103 ID FROM dual UNION ALL
SELECT to_date('01/01/2018', 'dd/mm/yyyy') active_dt, to_date('25/04/2018', 'dd/mm/yyyy') term_dt, 104 ID FROM dual)
SELECT active_month,
COUNT(*) num_active_ids
FROM (SELECT add_months(TRUNC(active_dt, 'mm'), -1 + LEVEL) active_month,
ID
FROM your_table
CONNECT BY PRIOR ID = ID
AND PRIOR sys_guid() IS NOT NULL
AND LEVEL <= FLOOR(months_between(coalesce(term_dt, SYSDATE), active_dt)) + 1)
GROUP BY active_month
ORDER BY active_month;
ACTIVE_MONTH NUM_ACTIVE_IDS
------------ --------------
01/01/2018 3
01/02/2018 3
01/03/2018 4
01/04/2018 4
01/05/2018 3
01/06/2018 2
01/07/2018 1
01/08/2018 1
01/09/2018 1
01/10/2018 1
Whether this is more or less performant than the other answers is up to you to test.