Oracle values change when row_number() added - oracle

Question

I have an Oracle query that uses DUAL to produce a list of dates in a subquery, and a case when to identify business days:
SELECT DATES
,case when to_char(DATES, 'd') in (1,7)
then 0
else 1 end as business_day
FROM (
SELECT to_date('1/1/2020','MM/DD/YYYY') + (LEVEL -1) AS DATES
FROM DUAL connect by level <=(to_date('1/1/2021','MM/DD/YYYY') - to_date('1/1/2020','MM/DD/YYYY'))
) L1
So far so good. Now when I nest this in a subquery, and add a row_number() function, all my business_day values become 0. If I remove the row_number() function, business_day goes back to normal.
SELECT L2.DATES
, L2.business_day
, row_number() OVER (PARTITION BY L2.business_day ORDER BY L2.DATES ASC) as dateindex
FROM (
SELECT DATES
,case when to_char(DATES, 'd') in (1,7)
then 0
else 1 end as business_day
FROM (
SELECT to_date('1/1/2020','MM/DD/YYYY') + (LEVEL -1) AS DATES
FROM DUAL connect by level <=(to_date('1/1/2021','MM/DD/YYYY') - to_date('1/1/2020','MM/DD/YYYY'))
) L1
) L2
Any idea how adding a new column causes another's values to change?

Answer 1

I suspect you aren't paying attention to the actual dates; running your code in this db<>fiddle, the first query returns:
DATES | BUSINESS_DAY
:-------- | -----------:
01-JAN-20 | 1
02-JAN-20 | 1
03-JAN-20 | 1
04-JAN-20 | 1
05-JAN-20 | 0
06-JAN-20 | 0
07-JAN-20 | 1
08-JAN-20 | 1
09-JAN-20 | 1
10-JAN-20 | 1
11-JAN-20 | 1
...
while the second returns:
DATES | BUSINESS_DAY | DATEINDEX
:-------- | -----------: | --------:
05-JAN-20 | 0 | 1
06-JAN-20 | 0 | 2
12-JAN-20 | 0 | 3
13-JAN-20 | 0 | 4
19-JAN-20 | 0 | 5
20-JAN-20 | 0 | 6
26-JAN-20 | 0 | 7
27-JAN-20 | 0 | 8
02-FEB-20 | 0 | 9
03-FEB-20 | 0 | 10
...
All the business_day values are indeed zero... or at least, if you only look at the start of the result set. If you look further down:
...
27-DEC-20 | 0 | 103
28-DEC-20 | 0 | 104
01-JAN-20 | 1 | 1
02-JAN-20 | 1 | 2
03-JAN-20 | 1 | 3
04-JAN-20 | 1 | 4
...
You don't have an order-by clause, and the analytic processing internally happens to return in an order you aren't expecting. If you add an order-by then it looks more sensible, as in this db<>fiddle:
DATES | BUSINESS_DAY | DATEINDEX
:-------- | -----------: | --------:
01-JAN-20 | 1 | 1
02-JAN-20 | 1 | 2
03-JAN-20 | 1 | 3
04-JAN-20 | 1 | 4
05-JAN-20 | 0 | 1
06-JAN-20 | 0 | 2
07-JAN-20 | 1 | 5
08-JAN-20 | 1 | 6
09-JAN-20 | 1 | 7
10-JAN-20 | 1 | 8
11-JAN-20 | 1 | 9
12-JAN-20 | 0 | 3
...
Incidentally, the 'd' format element is NLS-sensitive, so someone else running this code in a session with different settings could see different results. It would safer to do:
when to_char(DATES, 'Dy', 'NLS_DATE_LANGUAGE=ENGLISH') in ('Sat', 'Sun')