Oracle SQL get last year of dates excluding weekends - sql

Question

I'd expect this to work to get me a list of calendar dates over the past 12 months excluding weekends; but it just gives me the entire list of dates - which I suppose is fine - but want to know why the below is incorrect.
SELECT ADD_MONTHS(TRUNC(SYSDATE,'MM'),-12) - 1 + rownum AS CalendarDate
FROM all_objects
WHERE ADD_MONTHS(TRUNC(SYSDATE,'MM'),-12) - 1 + rownum <= sysdate
AND to_char(sysdate,'DY') NOT IN ('SAT','SUN')

Answer 1

Because you're doing this:
AND to_char(sysdate,'DY') NOT IN ('SAT','SUN')
And today isn't Saturday or Sunday. You need to look at the calculated CalendarDate value; but you can't do that in the same level of subquery. You could try to recalculate it:
AND to_char(ADD_MONTHS(TRUNC(SYSDATE,'MM'),-12) - 1 + rownum,'DY') NOT IN ('SAT','SUN')
but this will return no rows - at least when run at the moment. As it happens, March 1st 2020 was a Sunday, so that is excluded; and because of when and how rownum is generated, that result is excluded, and the next one sees the same value, which is excluded, and so on.
You can use an inline view to avoid both issues:
SELECT CalendarDate
FROM (
SELECT ADD_MONTHS(TRUNC(SYSDATE,'MM'),-12) - 1 + rownum AS CalendarDate
FROM all_objects
WHERE ADD_MONTHS(TRUNC(SYSDATE,'MM'),-12) - 1 + rownum <= sysdate
)
WHERE to_char(CalendarDate,'DY','NLS_DATE_LANGUAGE=ENGLISH') NOT IN ('SAT','SUN')
CALENDARDATE
02-MAR-20
03-MAR-20
04-MAR-20
05-MAR-20
06-MAR-20
09-MAR-20
10-MAR-20
...
db<>fiddle
I've chucked in a language modifier to stop it behaving differently for users with sessions not set to English.
Querying against all_objects isn't ideal though, it would be better to use a hierarcical query:
SELECT *
FROM (
SELECT ADD_MONTHS(TRUNC(SYSDATE,'MM'),-12) - 1 + level AS CalendarDate
FROM dual
CONNECT BY level <= TRUNC(SYSDATE) - ADD_MONTHS(TRUNC(SYSDATE,'MM'),-12) + 1
)
WHERE to_char(CalendarDate,'DY','NLS_DATE_LANGUAGE=ENGLISH') NOT IN ('SAT','SUN')
ORDER BY CalendarDate
db<>fiddle
or a recursive CTE, if you're 11gR2+:
WITH rcte (CalendarDate) AS (
SELECT ADD_MONTHS(TRUNC(SYSDATE,'MM'),-12)
FROM dual
UNION ALL
SELECT rcte.CalendarDate + interval '1' day
FROM rcte
WHERE rcte.CalendarDate < TRUNC(SYSDATE)
)
SELECT CalendarDate
FROM rcte
WHERE to_char(CalendarDate,'DY','NLS_DATE_LANGUAGE=ENGLISH') NOT IN ('SAT','SUN')
ORDER BY CalendarDate
db<>fiddle (as 18c to avoid a couple of issues with the patch level in the 11g version it uses).

Answer 2

You checking whether today is sunday or monday with to_char(sysdate,'DY'). you need to check CalendarDate which is not available in your window. You can use cte to calculate the calendar then you can remove weekends with your condition as below.
with cte (CalendarDate) as
(
SELECT ADD_MONTHS(TRUNC(SYSDATE,'MM'),-12) - 1 + rownum AS CalendarDate
FROM all_objects
WHERE ADD_MONTHS(TRUNC(SYSDATE,'MM'),-12) - 1 + rownum <= sysdate
)
select * from cte where
to_char(CalendarDate,'DY') not in ('SAT','SUN');
| CALENDARDATE |
| :----------- |
| 02-MAR-20 |
| 03-MAR-20 |
| 04-MAR-20 |
| 05-MAR-20 |
| 06-MAR-20 |
| 09-MAR-20 |
| 10-MAR-20 |
| 11-MAR-20 |
| 12-MAR-20 |
| 13-MAR-20 |
| 16-MAR-20 |
| 17-MAR-20 |
| 18-MAR-20 |
| 19-MAR-20 |
| 20-MAR-20 |
| 23-MAR-20 |
| 24-MAR-20 |
| 25-MAR-20 |
| 26-MAR-20 |
| 27-MAR-20 |
| 30-MAR-20 |
| 31-MAR-20 |
| 01-APR-20 |
| 02-APR-20 |
| 03-APR-20 |
| 06-APR-20 |
| 07-APR-20 |
| 08-APR-20 |
| 09-APR-20 |
| 10-APR-20 |
| 13-APR-20 |
| 14-APR-20 |
| 15-APR-20 |
| 16-APR-20 |
| 17-APR-20 |
| 20-APR-20 |
| 21-APR-20 |
| 22-APR-20 |
| 23-APR-20 |
| 24-APR-20 |
| 27-APR-20 |
| 28-APR-20 |
| 29-APR-20 |
| 30-APR-20 |
| 01-MAY-20 |
| 04-MAY-20 |
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db<>fiddle here