Format code cannot appear in date input format - IW related

Question

Hi,
please explain what I'm doing wrong:
SQL>  select to_char(sysdate, 'YYYY-IW') from  dual;
TO_CHAR
2010-48
SQL>  select to_date('2010-22', 'YYYY-IW') from  dual;
select to_date('2010-22', 'YYYY-IW') from  dual
ERROR at line 1:
ORA-01820: format code cannot appear in date input formatOracle 9.2 .
Regards.
Greg

Answer 1

FIrst of all you should stay with the date format. Instead of converting it into a string you could TRUNCATE it to the first day of the Iso-Wekk.
And to enable index usage you just have to put a certain range on your queries.
example
select *
from t
where col_date  >= trunc(sysdate, 'IW')  ;if col_date can hold values that are greater than syste. Or if you use another date value to compare than sysdate. Then you need to compare with a range. Just like this
select *
from t
where col_date  >= trunc(sysdate, 'IW') 
and col_date  < trunc(sysdate, 'IW')+7 ;If you want to input a certain WEEK and get all entries from that week you can start with the first day of the iso-Year and add the number of weeks to it.
example using iso-Week 40
select *
from t
where col_date  >= trunc(sysdate, 'IYYY')  + 7 * 40
and col_date  < trunc(sysdate, 'IYYY')+7 * (40 +  1);However calendar arithmetic is complex. We have to be careful for special cases. For example the beginning of the ISO-Year 2011 is the 3rd Jan 2011. So if we run this logic on the first or second of January *2011* we would get the calendar week 40 from *2010*. This is probably not what you want.
To take this into account we must make sure that we are using the proper year. There are several possibilities how to do it. In this case I choose to use the normal TRUNC to get the first day of the normal year (1.1.2011) then add a week and truncate again.
example
select *
from t
where col_date  >= trunc(trunc(sysdate,'YYYY)+7, 'IYYY')  + 7 * 40
and col_date  < trunc(trunc(sysdate,'YYYY)+7, 'IYYY')+7 * (40 +  1);Edited by: Sven W. on Dec 3, 2010 8:52 AM