Using DECODE() to insert to DATE field

Question

I'm trying to use the DECODE function to test for NULL before inserting to a DATE field. However, it seems to only insert the DATE, with a "default" time of 12:00 - it isn't properly inserting the time.
Basically I need to test if Date1 is NULL. If it isn't I need to concatenate the DATE from Date1 with the TIME from Date2 to get a full date/time... then insert this new value.
Generic Example:
CREATE TABLE DATETEST (TestID NUMBER(1), TestDate DATE);
DECLARE
v_Date1 DATE;
v_Date2 DATE;
BEGIN
v_Date1 := TO_DATE('01-JAN-11 05:53:12', 'DD-MON-YY HH:MI:SS');
v_Date2 := TO_DATE('08-FEB-11 02:18:31', 'DD-MON-YY HH:MI:SS');
INSERT INTO DATETEST (TestID, TestDate) VALUES ('1', DECODE(v_Date1, NULL, NULL, TO_DATE(To_Char(v_Date1, 'DD-MON-YY') || ' ' || TO_CHAR(v_Date2, 'HH:MI:SS'),'DD-MON-YY HH:MI:SS')));
INSERT INTO DATETEST (TestID, TestDate) VALUES ('2', TO_DATE(To_Char(v_Date1, 'DD-MON-YY') || ' ' || TO_CHAR(v_Date2, 'HH:MI:SS'),'DD-MON-YY HH:MI:SS'));
END;
SELECT TestID, TO_CHAR(TestDate, 'DD-MON-YY HH:MI:SS') from DATETEST;
This example performs two inserts. One with the DECODE function, and one without. The one without inserts the time properly. Can anyone tell me why the one with the DECODE function doesn't? I realize I can use a simple if/then to check if the date is null above and put the date/time in a variable, but since my real scenario is in a large chunk of other stuff, I'm trying to keep it as streamlined as possible.
Edited by: BoredBillJ on Jul 14, 2011 6:39 AM

Answer 1

The problem you are having is due to the nature of how DECODE and CASE determine what datatype to return, and you nls_date_format settings. Both use the data type of the first returnable argument to determine all of them. So, in your decode statement, the first returnable value is NULL which, in the absence of a cast (either implicit or explicit), is a varchar2 column. So, if the date is not null, the implicit conversion to a varchar to match the retunr type, then back to date to insert into the table is losing the time. you need something more like:
   INSERT INTO test_date (Test_ID, TestDate)
   VALUES ('1', DECODE(v_Date1, NULL, TO_DATE(NULL),
                                      TO_DATE(To_Char(v_Date1, 'DD-MON-YY') || ' ' ||
                                      TO_CHAR(v_Date2, 'HH:MI:SS'),'DD-MON-YY HH:MI:SS')));Even if you use Solomon's method of generating the date, if you need the decode/case, then you will have to either use the TO_DATE(NULL) or use case instead of decode and reverse the test so the first returnable is a date like:
SQL> DECLARE
  2     v_Date1 DATE;
  3     v_Date2 DATE;
  4  BEGIN
  5     v_Date1 := TO_DATE('01-JAN-11 05:53:12', 'DD-MON-YY HH:MI:SS');
  6     v_Date2 := TO_DATE('08-FEB-11 02:18:31', 'DD-MON-YY HH:MI:SS');
  7     INSERT INTO test_date (Test_ID, TestDate)
  8     VALUES ('1', CASE WHEN v_date1 IS NOT NULL
  9                       THEN TO_DATE(To_Char(v_Date1, 'DD-MON-YY') || ' ' ||
10                                    TO_CHAR(v_Date2, 'HH:MI:SS'),'DD-MON-YY HH:MI:SS')
11                       ELSE NULL END);
12     INSERT INTO test_date (Test_ID, TestDate)
13     VALUES ('2', TO_DATE(To_Char(v_Date1, 'DD-MON-YY') || ' ' ||
14                  TO_CHAR(v_Date2, 'HH:MI:SS'),'DD-MON-YY HH:MI:SS'));
15  END;
16  /
PL/SQL procedure successfully completed.
SQL> select test_id, to_char(testdate, 'dd-mon-yyyy hh24:mi:ss')
  2  from test_date;
   TEST_ID TO_CHAR(TESTDATE,'DD
         1 01-jan-2011 02:18:31
         2 01-jan-2011 02:18:31John