TO_DATE yy,yyyy gives me different results - plsql

Question

If I run the below queries
select to_char( TO_DATE(SYSDATE-7,'DD/MM/YY') ,'year') from dual
The result is twenty seventeen
select to_char( TO_DATE(SYSDATE-7,'DD/MM/YYYY') ,'year') from dual
The result is seventeen
why don't I get twenty seventeen for the second query?

Answer 1

Because you are doing an unnecessary explicit conversion from string to date, which is doing an implicit conversion from date to string based on your NLS settings.
What you should be doing is just:
select to_char(sysdate - 7, 'year') from dual;
TO_CHAR(SYSDATE-7,'YEAR')
------------------------------------------
twenty seventeen
Because sysdate-7 is already a date, you should not be calling to_date() around that, with any format. As the to_date() function takes a string argument, your sysdate-7 expression has to be implicitly converted to a string first. So you are really doing:
select to_char(to_date(to_char(sysdate - 7), 'DD/MM/YYYY'), 'year') from dual;
which is using your NLS_DATE_FORMAT value for the implicit inner to_char() call, so if that is say 'DD-MON-RR' you're actually doing:
select to_char(to_date(to_char(sysdate - 7, 'DD-MON-RR'), 'DD/MM/YYYY'), 'year') from dual;
To see what's going on in there you need to see what the full generated date is, so for that I'm going to change the NLS_DATE_FORMAT for the session to show the full year:
alter session set nls_date_format = 'SYYYY-MM-DD';
select sysdate - 7 as raw_date,
to_char(sysdate - 7, 'DD-MON-RR') as implicit_string,
to_date(to_char(sysdate - 7, 'DD-MON-RR'), 'DD/MM/YY') as implcit_yy,
to_date(to_char(sysdate - 7, 'DD-MON-RR'), 'DD/MM/YYYY') as implcit_yyyy
from dual;
RAW_DATE IMPLICIT_STRING IMPLCIT_YY IMPLCIT_YYY
----------- ------------------ ----------- -----------
2017-04-30 30-APR-17 2017-04-30 0017-04-30
Notice the different years in the last two values. The documentation has a section on format model matching. The string you are implicitly creating in the middle has a 2-digit year. When you convert the string '30-APR-17' back to a date using a YY model, it 'helpfully' assumes the current century, so the date ends up as 2017. But when you convert the same string using a YYYY model it thinks you really meant the value you passed in and so doesn't assume a century - and you passed it 17, so you end up with the year 17; that is, 0017 and not 2017. (You would also have got the answer you expected using RRRR, but it is much better to stick to YYYY and actually use 4-digit years everywhere - if you actually need to use a string at all.)
Essentially: don't rely on NLS settings or implicit conversions of dates to strings or vice versa.
In this case you don't need any conversions, so use the simpler statement at the top of this answer.

Answer 2

you should get 2017 for second query, not for the 1 query. Other possibility is NLS settings or session, software settings.
Apart from that you have to use 'YYYY' instead of 'YY' to get 2017