2020-02-29 减去一年导致错误 ORA-01839：指定的月份日期无效

Question

I have a simple query as:

select to_date('2020-02-29', 'yyyy-mm-dd') - interval '1' year from dual

I think the result should be 2019-02-28 , but oracle throws error as:

Error report -
ORA-01839: date not valid for month specified

Answer 1

That is the documented behaviour ; it even gives this as an example:

When interval calculations return a datetime value, the result must be an actual datetime value or the database returns an error. For example, the next two statements return errors:

 SELECT TO_DATE('31-AUG-2004','DD-MON-YYYY') + TO_YMINTERVAL('0-1') FROM DUAL; SELECT TO_DATE('29-FEB-2004','DD-MON-YYYY') + TO_YMINTERVAL('1-0') FROM DUAL;

The first fails because adding one month to a 31-day month would result in September 31, which is not a valid date. The second fails because adding one year to a date that exists only every four years is not valid. However, the next statement succeeds, because adding four years to a February 29 date is valid:

 SELECT TO_DATE('29-FEB-2004', 'DD-MON-YYYY') + TO_YMINTERVAL('4-0') FROM DUAL; TO_DATE(' --------- 29-FEB-08

The alternative is to use add_months(..., -12) ( docs ), which won't error:

select add_months(date '2020-02-29', -12) from dual;

ADD_MONTHS
----------
2019-02-28

But note how that deals with different number of days in the month; not really an issue when you're going back exactly a year, but still something to be aware of:

If date is the last day of the month or if the resulting month has fewer days than the day component of date , then the result is the last day of the resulting month.

So some of these might not do what you expected:

with rcte (dt) as (
  select last_day(date '2020-01-01')
  from dual
  union all
  select last_day(trunc(dt, 'MM') + interval '1' month)
  from rcte
  where dt < date '2020-06-01'
)
select dt,
  add_months(dt, -12) as minus12, add_months(dt, -3) as minus3, add_months(dt, -1) as minus1,
  add_months(dt, 1) as plus1, add_months(dt, 3) as plus3, add_months(dt, 12) as plus12
from rcte
order by dt;

DT         MINUS12    MINUS3     MINUS1     PLUS1      PLUS3      PLUS12    
---------- ---------- ---------- ---------- ---------- ---------- ----------
2020-01-31 2019-01-31 2019-10-31 2019-12-31 2020-02-29 2020-04-30 2021-01-31
2020-02-29 2019-02-28 2019-11-30 2020-01-31 2020-03-31 2020-05-31 2021-02-28
2020-03-31 2019-03-31 2019-12-31 2020-02-29 2020-04-30 2020-06-30 2021-03-31
2020-04-30 2019-04-30 2020-01-31 2020-03-31 2020-05-31 2020-07-31 2021-04-30
2020-05-31 2019-05-31 2020-02-29 2020-04-30 2020-06-30 2020-08-31 2021-05-31
2020-06-30 2019-06-30 2020-03-31 2020-05-31 2020-07-31 2020-09-30 2021-06-30