Oracle 10g accepting 5 digit year in a Date

问题

I managed to enter the date 21-Feb-12017 (I know it's not a correct date) in Oracle 10g into a date column. Oracle accepted the date fine. When I tried to select it back in SQL Developer, SQL Developer displayed it as NULL. But when I tried to retrieve the date thru java, I got the value back as how I inserted. Wondering what's going on because I could also see that Oracle converted a different 5 digit year into a 4 digit year. I entered 21-Feb-21019 and Oracle converted the year to 4581 while storing. I could even select this value in SQL developer.

I was wondering if it be possible to read the original date back e.g 21-Feb-21019 changed to 21-Feb-4581, so how to read 21-Feb-21019 instead of 21-Feb-4581.

回答1:

Oracle stores DATEs in tables using 7 bytes where the first 2 bytes are:

Century + 100
Year of century + 100

So the maximum date that can (technically) be stored is when those two bytes have the values 255 and 199 which would give the a year of 15599 (I'm ignoring that you could theoretically store 255 in the second byte as that opens up a whole heap of separate issues).

You can convert a raw value to a date using the DBMS_STATS.CONVERT_RAW_VALUE which means we can bypass the normal methods of creating dates and directly generate the byte values which will be stored.

This function is an example of that:

CREATE FUNCTION createDate(
  year   int,
  month  int,
  day    int,
  hour   int,
  minute int,
  second int
) RETURN DATE DETERMINISTIC
IS
  hex CHAR(14);
  d DATE;
BEGIN
  hex := TO_CHAR( FLOOR( year / 100 ) + 100, 'fm0X' )
      || TO_CHAR( MOD( year, 100 ) + 100, 'fm0X' )
      || TO_CHAR( month, 'fm0X' )
      || TO_CHAR( day, 'fm0X' )
      || TO_CHAR( hour + 1, 'fm0X' )
      || TO_CHAR( minute + 1, 'fm0X' )
      || TO_CHAR( second + 1, 'fm0X' );
  DBMS_OUTPUT.PUT_LINE( hex );
  DBMS_STATS.CONVERT_RAW_VALUE( HEXTORAW( hex ), d );
  RETURN d;
END;
/

Then if you have a date column you can insert values you are not normally allowed to insert:

CREATE TABLE table_name ( date_column DATE );

INSERT INTO table_name ( date_column )
VALUES ( DATE '2019-12-31' + INTERVAL '1:02:03' HOUR TO SECOND );

INSERT INTO table_name ( date_column ) VALUES ( createDate( 15599, 12, 31, 1, 2, 3 ) );

INSERT INTO table_name ( date_column ) VALUES ( createDate( 12017, 2, 21, 0, 0, 0 ) );

TO_CHAR does not work when the year exceeds the normal bounds of a date. To get the values stored in the table you can use DUMP to get a string containing the byte values or you can use EXTRACT to get the individual components.

SELECT DUMP( date_column ),
       TO_CHAR( date_column, 'YYYY-MM-DD' ) AS value,
       TO_CHAR( EXTRACT( YEAR FROM date_column ), 'fm00000' )
         || '-' || TO_CHAR( EXTRACT( MONTH  FROM date_column ), 'fm00' )
         || '-' || TO_CHAR( EXTRACT( DAY    FROM date_column ), 'fm00' )
         || ' ' || TO_CHAR( EXTRACT( HOUR   FROM CAST( date_column AS TIMESTAMP ) ), 'fm00' )
         || ':' || TO_CHAR( EXTRACT( MINUTE FROM CAST( date_column AS TIMESTAMP ) ), 'fm00' )
         || ':' || TO_CHAR( EXTRACT( SECOND FROM CAST( date_column AS TIMESTAMP ) ), 'fm00' )
         AS full_value
FROM table_name;

outputs:

DUMP(DATE_COLUMN)                 | VALUE      | FULL_VALUE          
:-------------------------------- | :--------- | :-------------------
Typ=12 Len=7: 120,119,12,31,2,3,4 | 2019-12-31 | 02019-12-31 01:02:03
Typ=12 Len=7: 255,199,12,31,2,3,4 | 0000-00-00 | 15599-12-31 01:02:03
Typ=12 Len=7: 220,117,2,21,1,1,1  | 0000-00-00 | 12017-02-21 00:00:00

db<>fiddle here

来源：https://stackoverflow.com/questions/59758473/oracle-10g-accepting-5-digit-year-in-a-date

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