## Interval Expressions

An interval expression yields a value of `INTERVAL`

`YEAR`

`TO`

`MONTH`

or `INTERVAL`

`DAY`

`TO`

`SECOND`

.

**interval_expression::=**
Description of the illustration interval_expression.gif
The `interval_value_expr`

can be the value of an `INTERVAL`

column or a compound expression that yields an interval value. Datetimes and intervals can be combined according to the rules defined in Table 2-6. 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. Because we do not know how many days ago the oldest order was placed, we specify the maximum value of 9 for the `DAY`

lading field precision:

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