The standard approach to interval month arithmetic is to increment MONTH and YEAR values as appropriate and retain the source value for DAY. This is a problem for the case when the target DAY value is smaller than the source DAY value from the source date.
For example, what approach must be taken to handle the result of adding one MONTH to a source DATE value of '1999-01-31'? Using the standard approach, the answer is '1999-02-31', but February 31 is not a valid date.
The behavior of ADD_MONTHS is equivalent to that of the ANSI SQL:2011 compliant operations DATE ± INTERVAL 'n' MONTH and TIMESTAMP ± INTERVAL 'n' MONTH with one important difference.
The difference between these two scalar arithmetic operations is their behavior when a non valid date value is returned by the function.
- ANSI SQL:2011 arithmetic returns an error.
- ADD_MONTHS arithmetic makes normative adjustments and returns a valid date.