How to get Year/Week according to ISO?

Question

I have the below code, which gives week 202153. However, I would need to have, instead of the first week of year 2021 as 202153, as 202053.

Is there any built-in functionality to address this?

SELECT
    to_char(TO_DATE('20210104', 'YYYYMMDD') - 3, 'YYYYIW') AS yearweek
FROM
    dual

Expected output: 202053. I could simply decode the value, but I'm hoping for a more robust solution for future years.

Answer 1

Use IYYY (ISO-year) rather than the YYYY (calendar year) format model.

SELECT TO_CHAR( DATE '2021-01-04' - 3, 'IYYYIW') AS yearweek
FROM   DUAL

Which outputs:

YEARWEEK
202053

db<>fiddle here