Date confusion - need help

Question

Hi ,
my data's date(DATA_ENTRY_TIME) is --> 7/29/2006 4:02:02 AM
ques1 :
when i run this query --> select TO_DATE(DATA_ENTRY_TIME , 'dd-mon-rr HH.MI.SS AM' ) FROM LOT_HISTORY_2DCHRT
result : 7/29/2006
shldn't it return the time as well ?
ques 2 :
i have a insert query from a selection of such criteria so as to be the same format as the DATA_ENTRY_TIME
-->
WHERE DATA_ENTRY_TIME >= TO_CHAR(TRUNC(SYSDATE - 6 / 24) , 'MM/DD/YYYY HH:MI:SS AM')
AND DATA_ENTRY_TIME<= TO_CHAR(TRUNC(SYSDATE) , 'MM/DD/YYYY HH:MI:SS AM')
and i got the error while running it
ORA-01843: not a valid month
ORA-02063: preceding line from PLL2
what is wrong here actually ?
ques 3
select * from nls_session_parameters
where parameter in ('NLS_DATE_FORMAT' , 'NLS_TIME_FORMAT') ;
these are the values
DD-MON-RR & HH.MI.SSXFF AM
do i need to an alter session in order for ques 2 to work ??
ques 4
if both field are of date type but different format e.g field1 is 'dd/mm/yyyy hh:mi:ss am' and field2 is 'mon-dd-yyyy hh:mi:ss am'
can comparision such as field1 >= field2 still be done ?
ques 5
if i use to_char for both field1 & field2 in ques 4 , can a comparison be done and is it accurate as it's now a string(if i am correct) ?
tks & rdgs

Answer 1

my data's date(DATA_ENTRY_TIME) is --> 7/29/2006
4:02:02 AM
ques1 :
when i run this query --> select
TO_DATE(DATA_ENTRY_TIME , 'dd-mon-rr HH.MI.SS AM' )
FROM LOT_HISTORY_2DCHRT
result : 7/29/2006
shldn't it return the time as well ?What you see depends on the settings you have for displaying the data. You can change your default date format settings as follows:
SQL> select sysdate from dual;
SYSDATE
08/02/2006
SQL> alter session set nls_date_format='mm/dd/yyyy hh24:mi:ss';
Session altered.
SQL> select sysdate from dual;
SYSDATE
08/02/2006 08:08:44
SQL>Or you can muck about with converting the dates into varchars if that's what you really want, but then if you need to do further date functionality against them you have to ensure you convert them correctly back to date format. I'd suggest that you just work with dates as oracle designed them to be and only convert to varchar (using TO_CHAR) when you really really need to.
ques 4
if both field are of date type but different format
e.g field1 is 'dd/mm/yyyy hh:mi:ss am' and field2 is
'mon-dd-yyyy hh:mi:ss am'
can comparision such as field1 >= field2 still be
done ?Internally the date types are stored identically. The formatting only comes into play when dates are displayed or converted to varchar so yes, if they are both stored on the database as DATE types then you can just do a straight comparison. I'm not sure how you are determining that they are "different format" unless you are querying them in different environments.
>
ques 5
if i use to_char for both field1 & field2 in ques 4 ,
can a comparison be done and is it accurate as it's
now a string(if i am correct) ?If you convert to a varchar then you are doing a string comparison which works fine if you are doing "=", but if you are intending to do ">=" then you'll likely get some unexpected results
SQL> select 'greater'
  2  from dual
  3  where to_date('08/02/2006 08:00:00', 'mm/dd/yyyy hh24:mi:ss') >= to_date('08/10/2005 08:00:00', 'mm/dd/yyyy hh24:mi:ss');
'GREATE
greater
SQL> ed
Wrote file afiedt.buf
  1  select 'greater'
  2  from dual
  3* where '08/02/2006 08:00:00' >= '08/10/2005 08:00:00'
SQL> /
no rows selected
SQL>The second comparison using varchar doesn't work because the 4th character in the right hand string ("1") is greater than the 4th character in the left hand string ("0"). It is literally a chr for chr comparison rather than treating the whole string as a value in its own right which is what you get with dates.
For simplicity's sake, think of dates as being stored on the database as the number of seconds since a fixed date in time. As they are purely stored as a number internally it's easy for the database to compare them. The moment you conver things to varchar you just get yourself in a mess.
;)