When a+b+c is not equal to a+c+b

Question

Hi,
I would be pleased to have some comment on the following :
if a+b+c != a+c+b then something the + operator is not working as it should !?
SQL> select
a,b,c,a+b+c,a+c+b
from
(select date '2005-02-28' a, numtoyminterval(1,'MONTH') b, numtodsinterval(1,'DAY') c from dual);
A          B             C                   A+B+C      A+C+B
28.02.2005 +000000000-01 +000000001 00:00:00 29.03.2005 01.04.2005

Answer 1

Hi Laurent
As James pointed out, the behaviour with add_months is different but gives a consistent result for the 2 orders. This I can understand because of the functionality and the additional logic that can be placed in the function call.
SQL> select
  2     a+b+c,
  3     a+c+b,
  4     a+b,
  5     a+c,
  6     add_months(a,1) + 1 abc,
  7     add_months(a+1,1) acb
  8  from
  9     (select
10             date '2005-02-28' a,
11             numtoyminterval(1,'MONTH') b,
12             numtodsinterval(1,'DAY') c
13     from
14             dual
15     );
A+B+C     A+C+B     A+B       A+C       ABC       ACB
29-MAR-05 01-APR-05 28-MAR-05 01-MAR-05 01-APR-05 01-APR-05
SQL> select
  2     date '2005-03-29' - numtoyminterval(1,'MONTH')
  3  from
  4     dual;
        date '2005-03-29' - numtoyminterval(1,'MONTH')
ERROR at line 2:
ORA-01839: date not valid for month specified
SQL>
SQL> select
  2     add_months(date '2005-03-29',-1)
  3  from
  4     dual;
ADD_MONTH
28-FEB-05So based on this, I don't see how the + operator could be commutative for anything other than numbers. Dates and intervals are a special case where order of evaluation does (and should?) effect the result...aren't they?