Conversion from DATE to TIMESTAMP datatype

Question

Hello,
My issue is as follows:
1. I have one variable of type DATE, which I assign the value of SYSDATE
mydatevar DATE:= SYSDATE;2. I want to find *"today"*, truncated to DAY
  TRUNC (mydatevar, 'DD')
TRUNC function returns DATE datatype. So I will receive in point 2 for example *'2010-01-13 00:00:00'*.
3. I want to assign the value from point 2 to a variable of type TIMESTAMP
  mytimestampvar TIMESTAMP := mydatevar;which implicitly will convert the DATE variable to TIMESTAMP.
Problem: During the conversion (both implicit and explicit conversion with a format mask) I lose the "00" hours and "00" minutes and receive something like this: "10-JAN-13 *12*.00.00.000000000 AM".
Question: How can I convert from DATE to TIMESTAMP keeping hours and minutes zeros?
Why I need this conversion: I have a table with a column "column1" TIMESTAMP(0) and I would like to take only those rows from the table, where "column1" is in range from today 12 o'clock in the morning till now (whatever hour it is).
NLS characteristics of the database:
PARAMETER                           VALUE
NLS_LANGUAGE                           AMERICAN
NLS_TERRITORY                   AMERICA
NLS_CURRENCY     $
NLS_ISO_CURRENCY                    AMERICA
NLS_NUMERIC_CHARACTERS     .,
NLS_CHARACTERSET                    AL32UTF8
NLS_CALENDAR                            GREGORIAN
NLS_DATE_FORMAT                    DD-MON-RR
NLS_DATE_LANGUAGE            AMERICAN
NLS_SORT     BINARY
NLS_TIME_FORMAT                     HH.MI.SSXFF AM
NLS_TIMESTAMP_FORMAT             DD-MON-RR HH.MI.SSXFF AM
NLS_TIME_TZ_FORMAT             HH.MI.SSXFF AM TZR
NLS_TIMESTAMP_TZ_FORMAT     DD-MON-RR HH.MI.SSXFF AM TZR
NLS_DUAL_CURRENCY              $
NLS_COMP                             BINARY
NLS_LENGTH_SEMANTICS               BYTE
NLS_NCHAR_CONV_EXCP             FALSE
NLS_NCHAR_CHARACTERSET     AL16UTF16
NLS_RDBMS_VERSION             10.2.0.4.0Session parameters are the same.
DBTIMEZONE is "+02:00".

Answer 1

Verdi wrote:
Problem: During the conversion (both implicit and explicit conversion with a format mask) I lose the "00" hours and "00" minutes and receive something like this: "10-JAN-13 *12*.00.00.000000000 AM".I don't think you are necessarily losing any information whatsoever. It's probably more of a function of your NLS_TIMESTAMP_FORMAT and NLS_DATE_FORMAT. For example your NLS_DATE_FORMAT could be setup by default for a HH24 (24 hour time) which would report midnight as "00" hours. However, it looks like your NLS_TIMESTAMP_FORMAT is setup with a "HH" format with a meridian indicator which means 12 hours time.
Your comparisons should be using date/timestamp data types anyways so as long as the input value is converted properly into a date type this shouldn't matter anyways.
You can see what is actually stored by using the DUMP function:
SQL> SELECT  DUMP(TO_TIMESTAMP(TO_CHAR(TRUNC(SYSDATE,'DD'),'MM/DD/YYYY HH:MI:SS AM'))) AS TSTAMP
  2  ,       DUMP(TRUNC(SYSDATE,'DD')) AS DT
  3  FROM DUAL
  4  /
TSTAMP                                                                      DT
Typ=187 Len=20: 218,7,1,13,0,0,0,0,0,0,0,0,0,0,3,0,0,0,0,0                  Typ=13 Len=8: 218,7,1,13,0,0,0,0As you can see the TSTAMP and DT store nearly the same values (218,7,1,13), but the TSTAMP has more precision because of fractional seconds.
HTH!
Edited by: Centinul on Jan 13, 2010 7:23 AM