Oracle SQL Get date from year and week number - sql

Question

I have year and week numbers in a SQL table in one column with a format as VERSION_YEAR_WEEKNUMBER
I can use the SUBSTR function to just get the YEAR_WEEKNUMBER, but I then need to convert that to a date and I'm not sure how to convert a weeknumber to a date.
Ideally the date would be a Thursday, but right now I'd just like to convert a weeknumber to any date that's in that week and year combination.
Sample data would just be "VERSION_YEAR_WEEKNUMBER" e.g. VERSION_2020_10. Meaning, 2020, 10th week (ISO) of the year. Desired result should be one date between 02-MAR-20 and 08-MAR-20

Answer 1

You can extract the year and week numbers separately:
substr(your_column, instr(your_column, '_', -1, 2) + 1, 4)
substr(your_column, instr(your_column, '_', -1, 1) + 1)
Then convert the year number to the first day of the ISO year:
trunc(to_date(substr(your_column, instr(your_column, '_', -1, 2) + 1, 4), 'YYYY'), 'IYYY')
and either add the number of weeks - 1 * 7 days to get the Monday of that week, then add 3 for the Thursday; or add the number of weeks * 7 days to get the Monday of the following week and subtract four for the Thursday of the week you want:
trunc(to_date(substr(your_column, instr(your_column, '_', -1, 2) + 1, 4), 'YYYY'), 'IYYY')
+ (to_number(substr(your_column, instr(your_column, '_', -1, 1) + 1)) * 7)
- 4
YOUR_COLUMN RESULT
--------------- ----------
VERSION_2020_10 2020-03-05
1.2.3_2021_01 2021-01-07
1_2_3_2020_11 2020-03-12
2020_12 2020-03-19
ABC_2020_52 2020-12-24
DEF_2020_53 2020-12-31
GHI_2021_01 2021-01-07
JKL_2021_52 2021-12-30
MNO_2021_53 2022-01-06
db<>fiddle showing the intermediate steps so you can see what's happening.

Answer 2

Converting from ISO-Week to year is not trival, because the ISO year may differ from the actual year.
For example 2018-12-31 was Week 1 of 2019 according to ISO-8601.
I ended up with this function:
FUNCTION ISOWeek2Date(YEAR INTEGER, WEEK INTEGER) RETURN DATE DETERMINISTIC IS
res DATE;
BEGIN
IF WEEK > 53 OR WEEK < 1 THEN
RAISE VALUE_ERROR;
END IF;
res := NEXT_DAY(TO_DATE( YEAR || '0104', 'YYYYMMDD' ) - 7, 'MONDAY') + ( WEEK - 1 ) * 7;
IF TO_CHAR(res, 'fmIYYY') = YEAR THEN
RETURN res;
ELSE
RAISE VALUE_ERROR;
END IF;
END ISOWeek2Date;
However NEXT_DAY depends on date language of your session, so the solution by #MT0 would be better. The logic is the same.
Extract year and week values with regular expression as proposed by #Alex Poole

Answer 3

Converting to an ISO week is relatively simple as the 4th of January will always be in the first ISO week of the year. So:
Start with the 4th January of your year;
Truncate it back to Monday to be the start of the ISO week;
Add 3 days and you have Thursday of the first ISO week; and then
Add the number of weeks to take it from the 1st week to the required week.
Like this:
SELECT TRUNC( TO_DATE( SUBSTR( value, 9, 4 )||'01-04', 'YYYY-MM-DD' ), 'IW' )
+ INTERVAL '3' DAY
+ INTERVAL '7' DAY * ( SUBSTR( value, 14 ) - 1 )
AS thursday_of_iso_week
FROM table_name
Which, for the sample data:
CREATE TABLE table_name ( value ) AS
SELECT 'VERSION_2020_10' FROM DUAL UNION ALL
SELECT 'VERSION_2019_1' FROM DUAL
Outputs:
| THURSDAY_OF_ISO_WEEK |
| :------------------- |
| 2020-03-05 00:00:00 |
| 2019-01-03 00:00:00 |
If the values do not have fixed character positions then use INSTR to find the underscores:
SELECT TRUNC( TO_DATE( SUBSTR( value, INSTR( value, '_' ) + 1, 4 )||'01-04', 'YYYY-MM-DD' ), 'IW' )
+ INTERVAL '3' DAY
+ INTERVAL '7' DAY * ( SUBSTR( value, INSTR( value, '_', -1 ) + 1 ) - 1 )
AS thursday_of_iso_week
FROM table_name
Which outputs the same as above.
db<>fiddle here

Answer 4

You could join the following CTE to your query
select weekdate, to_char(weekdate, 'IW-yyyy') week_year from (
SELECT LEVEL weeknr, to_date('2020-01-02', 'yyyy-MM-dd') + numtodsinterval((LEVEL-1)*7, 'day') weekdate
FROM dual
CONNECT BY LEVEL <= 53
);
For the join, use the result of your SUBSTR and WEEK_YEAR of the CTE. WEEKDATE would be the date. This CTE generates all the week numbers for all Thursdays of 2020 (January 2nd was the first Thursday in 2020). If you need more dates, just use another limit for "LEVEL". If you have huge amounts of source data or query the data very frequently, I would recommend to put this in a table (to be done one time) by appending "create table ... as" at the beginning and create an index on WEEK_YEAR (and generating enough data so your solution works for a long time).