How to do a WHERE cause with dates

Question

Hello!
I'm new to Oracle and I'm wondering how to select records like this: WHERE (YEAR(SOME_DATE) > YEAR(NOW()) - 3).
YEAR in T-SQL returns an Int, so I can do math pretty easy. How would one do this with TO_CHAR? Do I have to cast to an Int or what?
Thanks!

Answer 1

Hi,
So, when you do this in 2009, you want to know if dt is in 2006 or later, and when you do the same thing in 2010, you'll want to know if dt is 2007 or later; is that it?
Date arithmetic in Oracle was designed for counting days. For dealing with larger units, ADD_MONTHS is the best thing.
SYSDATE is today's date
ADD_MONTHS (SYSDATE, -36) is exactly 3 years ago
TRUNC (ADD_MONTHS (SYSDATE, -36), 'YYYY') is the beginning of the year 3 years ago, so
WHERE   dt >= TRUNC (ADD_MONTHS (SYSDATE, -36), 'YYYY') will be TRUE if dt is any time in 2006 or later, given that it is currently 2009.
By not applying any functions or date arithmetic to dt, we allow the optimizer to use an index on it.