Oracle: not a valid month - sql

Question

I have a table with the following fields:
Reports (table name)
Rep_Date (date)
Rep_Time (date)
The Rep_Time field has values like '01/01/1753 07:30:00' i.e. the time part is relevant. I have written the following query:
select Reports.pid, MaxDate from Reports
INNER JOIN (
select pid, max(TO_DATE(TO_CHAR(REP_DATE, 'DD/MM/YYYY')
|| TO_CHAR(REP_TIME, 'HH24:MI:SS'), 'DD/MM/YYYY HH24:MI:SS')) As MaxDate
from reports
group by pid
) ReportMaxDate
on Reports.PID = ReportMaxDate.PID
AND To_Date(To_Char(MaxDate, 'DD/MM/YYYY')) = REP_DATE
WHERE REPORTS.PID=61
The derived table part of the query runs, but when I run the entire query I get an error: "not a valid month". Why is this?
In order to help debug this; if I run the following query:
select rep_date, rep_time from reports where pid=61 and rownum=1
I get:
Rep_Date = 01/04/2009
Rep_Time = 01/01/1753 13:00:00
UPDATE 15:58
I am now able to execute the following query:
select Reports.pid, MaxDate from Reports
INNER JOIN (
select pid, max(TO_DATE(TO_CHAR(REP_DATE, 'DD/MM/YYYY')
|| TO_CHAR(REP_TIME, 'HH24:MI:SS'), 'DD/MM/YYYY HH24:MI:SS')) As MaxDate
from reports group by pid
) ReportMaxDate
on Reports.PID = ReportMaxDate.PID
AND to_date(to_char(maxdate,'MM/DD/YYYY'),'MM/DD/YYYY') = REP_DATE
WHERE REPORTS.PID=61
However, I need to add one more statement to the WHERE clause comparing the time part of MaxDate to rep_time: to_date(to_char(maxdate,'MM/DD/YYYY'),'MM/DD/YYYY') = REP_DATE does not work.

Answer 1

1.
To_Date(To_Char(MaxDate, 'DD/MM/YYYY')) = REP_DATE
is causing the issue. when you use to_date without the time format, oracle will use the current sessions NLS format to convert, which in your case might not be "DD/MM/YYYY". Check this...
SQL> select sysdate from dual;
SYSDATE
---------
26-SEP-12
Which means my session's setting is DD-Mon-YY
SQL> select to_char(sysdate,'MM/DD/YYYY') from dual;
TO_CHAR(SY
----------
09/26/2012
SQL> select to_date(to_char(sysdate,'MM/DD/YYYY')) from dual;
select to_date(to_char(sysdate,'MM/DD/YYYY')) from dual
*
ERROR at line 1:
ORA-01843: not a valid month
SQL> select to_date(to_char(sysdate,'MM/DD/YYYY'),'MM/DD/YYYY') from dual;
TO_DATE(T
---------
26-SEP-12
2.
More importantly, Why are you converting to char and then to date, instead of directly comparing
MaxDate = REP_DATE
If you want to ignore the time component in MaxDate before comparision, you should use..
trunc(MaxDate ) = rep_date
instead.
==Update : based on updated question.
Rep_Date = 01/04/2009 Rep_Time = 01/01/1753 13:00:00
I think the problem is more complex. if rep_time is intended to be only time, then you cannot store it in the database as a date. It would have to be a string or date to time interval or numerically as seconds (thanks to Alex, see this) . If possible, I would suggest using one column rep_date that has both the date and time and compare it to the max date column directly.
If it is a running system and you have no control over repdate, you could try this.
trunc(rep_date) = trunc(maxdate) and
to_char(rep_date,'HH24:MI:SS') = to_char(maxdate,'HH24:MI:SS')
Either way, the time is being stored incorrectly (as you can tell from the year 1753) and there could be other issues going forward.

Answer 2

To know the actual date format, insert a record by using sysdate. That way you can find the actual date format. for example
insert into emp values(7936, 'Mac', 'clerk', 7782, sysdate, 1300, 300, 10);
now, select the inserted record.
select ename, hiredate from emp where ename='Mac';
the result is
ENAME HIREDATE
Mac 06-JAN-13
voila, now your actual date format is found.

Answer 3

You can also change the value of this database parameter for your session by using the ALTER SESSION command and use it as you wanted
ALTER SESSION SET NLS_DATE_FORMAT = 'DD-MM-YYYY';
SELECT TO_DATE('05-12-2015') FROM dual;
05/12/2015

Answer 4

You can customize as you wish the format inside the ''
SELECT TO_CHAR(column_name, 'MONTH-DAY-yy') FROM table_name;