A scalar function takes input parameters and creates a result.
When it is part of an expression, the function is invoked whenever expressions are evaluated for an SQL statement. When the function completes, the expression uses its result.
For example, the following request returns the current date plus 13 years.
SELECT ADD_MONTHS (CURRENT_DATE, 12*13);
The following request returns the date 6 months ago.
SELECT ADD_MONTHS (CURRENT_DATE, -6);