MAX function to get recent date doesn't work as expected

Question

Hi,
I recently started working on Oracle and came across this problem. I coded below given query to get the latest date from table and compare it with Sysdate + 1 timestamp. I am not able to understand what I coded wrong but the query gives me wrong results as explained below :
select NVL(MAX(to_char(max(last_update),'mm/dd/yyyy hh12:mi:ss AM')), to_char(sysdate + 1,'mm/dd/yyyy')||' 12:00:00 AM')
from Audit_Table_Name
where col1 = 'AA'
and location_type = 'STATE'
and original_flag = 'Y'
group by col1,location_type,original_flag;
Here Max(last_update) from Audit Table Contains value --> 08/25/2009 12:00:00 AM
Note : Data type of last_update is Date
so ideally speaking it should give 08/26/2009 12:00:00 AM but surprisingly it is giving 08/25/2009 12:00:00 AM as output. Anyone can explain why this is happening and what we need to do to get correct results?
Regards,
Amol

Answer 1

1) Why would you expect this to return a date of 8/26? NVL returns the first parameter unless it is NULL. If the first parameter is NULL, it returns the second parameter. Since
MAX(to_char(max(last_update),'mm/dd/yyyy hh12:mi:ss AM'))returns a non-NULL value, that value will be returned. Oracle never needs to evaluate the second parameter
to_char(sysdate + 1,'mm/dd/yyyy')||' 12:00:00 AM'You state that you want to compare the MAX( last_update ) to SYSDATE + 1. Your code isn't doing a comparison now. And it's not clear from your description what comparison you actually want to do. Can you elaborate a bit on what you want to compare and what the output should be depending on the result of that comparison?
2) You almost certainly want to do as much of the logic as possible using dates and only convert to a string at the very end. Otherwise, you risk comparing two strings that represent dates using string comparison semantics and getting a result you don't expect.
3) My guess (and it is just a guess) is that you want something like
SELECT GREATEST( max(last_update), trunc(sysdate+1) )or
SELECT TO_CHAR( GREATEST( max(last_update), trunc(sysdate+1) ), 'mm/dd/yyyy hh12:mi:ss AM' )Justin