An interval expression yields a value of INTERVAL YEAR TO MONTH or INTERVAL DAY TO SECOND.

interval_expression::=

The expressions expr1 and expr2 can be any expressions that evaluate to values of data type DATE, TIMESTAMP, TIMESTAMP WITH TIME ZONE, or TIMESTAMP WITH LOCAL TIME ZONE.

Datetimes and intervals can be combined according to the rules defined in Table 2-5. The six combinations that yield interval values are valid in an interval expression.

Both leading_field_precision and fractional_second_precision can be any integer from 0 to 9. If you omit the leading_field_precision for either DAY or YEAR, then Oracle Database uses the default value of 2. If you omit the fractional_second_precision for second, then the database uses the default value of 6. If the value returned by a query contains more digits that the default precision, then Oracle Database returns an error. Therefore, it is good practice to specify a precision that you know will be at least as large as any value returned by the query.

For example, the following statement subtracts the value of the order_date column in the sample table orders (a datetime value) from the system timestamp (another datetime value) to yield an interval value expression. It is not known how many days ago the oldest order was placed, so the maximum value of 9 for the DAY leading field precision is specified:

SELECT (SYSTIMESTAMP - order_date) DAY(9) TO SECOND FROM orders
   WHERE order_id = 2458;