Oracle SQL: Extracting the week of the year from date gives random results

Question

I have been using the following statement to get the week of the year from a date:

TO_CHAR(TO_DATE(tbl.col,'YYYY/MM/DD'),'IW') AS week

Now I'm receiving strange results and I can't seem to figure this out. For the following dates I receive the week 24:

19-Jun-17 || 23-Jun-17 || 24-Jun-17 || 25-Jun-17 || 29-Jun-17 || 30-Jun-17

For these, I receive 25:

20-Jun-17 || 21-Jun-17 || 22-Jun-17 || 26-Jun-17 || 27-Jun-17 || 28-Jun-17

...and this continues. I'm a bit worried now that all my queries show incorrect data. The only reason I have noticed it only now is because there are no results for weeks 26, 27, 30, 31 and 32 which is not the case in any of my other queries.

Any suggestions or pointers towards possible errors are appreciated.

Thank you!

Answer 1

[TL;DR] Just use: TO_CHAR( tbl.col, 'IW' )

You have multiple issues:

1. The datatype is "date"

   You are calling TO_DATE( string, format_model ) with a DATE (not a VARCHAR2 ) which causes Oracle to do an implicit conversion of the DATE to a VARCHAR2 using the NLS_DATE_FORMAT session parameter as the format model just so you can convert it back to a DATE . So, you are effectively doing:

    TO_CHAR( TO_DATE( TO_CHAR( tbl.col, ( SELECT VALUE FROM NLS_SESSION_PARAMETERS WHERE PARAMETER = 'NLS_DATE_FORMAT' ) ), 'YYYY/MM/DD' ), 'IW' ) 

   Do not do this , just use:

    TO_CHAR( tbl.col, 'IW' ) 

   If you do that then the following issues are irrelevant.

2. As RealCheeseLord points out, the position of year and day in your format model is reversed; and

3. You are using YYYY for the year format model so all the years are going to be in the 1st Century AD.

   Example :

    SELECT TO_CHAR( TO_DATE( '19-Jun-17', 'YYYY/MM/DD' ), 'YYYY-MM-DD' ) AS dt FROM DUAL 

   Output :

    DT ---------- 0019-06-17 

   If you aren't going to fix the implicit string conversion then you would probably want either:

    TO_CHAR(TO_DATE(tbl.col,'DD-MON-YY'),'IW') AS week 

   or (depending on whether you want the year 99 to be 1999 or 2099):

    TO_CHAR(TO_DATE(tbl.col,'DD-MON-RR'),'IW') AS week 

4. You are using the MM format model for MON formatted data - this is not necessarily an issue as MM also matches MON and MONTH but you should probably use the correct model.

Answer 2

You have the wrong order of year and day. those weeks are correct for example 17.06.2020 is week 25, 17.06.2023 is week 24 and so on.

Try: TO_CHAR(TO_DATE(tbl.col,'DD-MON-YY'),'IW') AS week

or something

examples at bottom of page