How to replace DB2 WEEK function to Oracle function? - oracle

Question

I am trying to replace DB2 with Oracle DB.
In DB2, there is a WEEK function, which returns the number of weeks of the year.
For example:
SELECT week('2021-01-04') FROM sysibm.sysdummy1
Then I get the return of 2 (as DB2 WEEK function regards Sunday as the first day of the week.)
However, in Oracle, if I make a similar query, then I get a different value.
SELECT to_char(to_date('2021-01-04', 'YYYY-MM-DD'), 'WW') FROM dual;
I get the value of 1 (not 2 as Oracle regards January 1st as the first day of the week.)
Is there any other turnaround or different function to replace DB2 WEEK function?

Answer 1

Oracle offers two Week of Year Number functions. As you have discovered, to_char(dt, 'WW') gives a number in which the week starts on 1st of January and increments every seven days. There is also to_char(dt, 'IW') giving the ISO week number, which runs Monday to Sunday; in this case the 1st of January is Week 53 and 2021-01-04 is the first day of week 1 of 2021.
Demo on db<>fiddle
There is no function in Oracle which increments the week number on the basis of the day of the week number. You could write your own PL/SQL function to do this.
Incidentally, the first day of the week in Oracle is determined by our NLS parameters. If we run with (say) American settings Sunday is day 1; if we run with British settings then Monday is day 1.

Answer 2

If you want to count the number of weeks where the first day of the week is Sunday then:
SELECT FLOOR(
(DATE '2021-01-04' - NEXT_DAY( TRUNC( DATE '2021-01-04', 'YY' ) - INTERVAL '7' DAY, 'SUNDAY'))
/ 7
) + 1
AS week
FROM your_input;
Which outputs 2.
If you run it with multiple input days:
WITH your_input ( value ) AS (
SELECT DATE '2021-01-01' + LEVEL - 1
FROM DUAL
CONNECT BY LEVEL <= 32
)
SELECT value,
FLOOR(
(value - NEXT_DAY( TRUNC( value, 'YY' ) - INTERVAL '7' DAY, 'SUNDAY'))
/ 7
) + 1
AS week
FROM your_input;
Then you get the output (with the NLS_DATE_FORMAT as YYYY-MM-DD (DY)):
VALUE
WEEK
2021-01-01 (FRI)
1
2021-01-02 (SAT)
1
2021-01-03 (SUN)
2
2021-01-04 (MON)
2
2021-01-05 (TUE)
2
2021-01-06 (WED)
2
2021-01-07 (THU)
2
2021-01-08 (FRI)
2
2021-01-09 (SAT)
2
2021-01-10 (SUN)
3
2021-01-11 (MON)
3
2021-01-12 (TUE)
3
2021-01-13 (WED)
3
2021-01-14 (THU)
3
2021-01-15 (FRI)
3
2021-01-16 (SAT)
3
2021-01-17 (SUN)
4
2021-01-18 (MON)
4
2021-01-19 (TUE)
4
2021-01-20 (WED)
4
2021-01-21 (THU)
4
2021-01-22 (FRI)
4
2021-01-23 (SAT)
4
2021-01-24 (SUN)
5
2021-01-25 (MON)
5
2021-01-26 (TUE)
5
2021-01-27 (WED)
5
2021-01-28 (THU)
5
2021-01-29 (FRI)
5
2021-01-30 (SAT)
5
2021-01-31 (SUN)
6
2021-02-01 (MON)
6
db<>fiddle here