How to find difference b/w TIMESTAMP format values in Oracle?

Question

I have two timestamp columns: arrTime and depTime. I need to find the number of munites the bus is late. I tried the following:

SELECT RouteDate, round((arrTime-depTime)*1440,2) time_difference
FROM ...

I get the following error: inconsistent datatype . expected number but got interval day to second

How can i parse the nuber of minutes?

If i simply subtract: SELECT RouteDate, arrTime-depTime)*1440 time_difference The result is correct but not well formatted:

 time_difference
  +00000000 00:01:00 0000000

Answer 1

The result of timestamp arithmetic is an INTERVAL datatype. You have an INTERVAL DAY TO SECOND there...

If you want the number of minutes one way would be to use EXTRACT(), for instance:

select extract( minute from interval_difference ) * 60 
        + extract( hour from interval_difference ) * 60
        + extract( day from interval_difference ) * 60 * 24
  from ( select systimestamp - (systimestamp - 1) as interval_difference
           from dual )

Alternatively you can use a trick with dates:

select sysdate + (interval_difference * 1440) - sysdate
  from (select systimestamp - (systimestamp - 1) as interval_difference
          from dual )

The "trick" version works because of the operator order of precedence and the differences between date and timestamp arithmetic.

Initially the operation looks like this:

date + ( interval * number ) - date

As mentioned in the documentation:

Oracle evaluates expressions inside parentheses before evaluating those outside.

So, the first operation performed it to multiply the interval by 1,440. An interval, i.e. a discrete period of time, multiplied by a number is another discrete period of time, see the documentation on datetime and interval arithmetic. So, the result of this operation is an interval, leaving us with:

date + interval - date

The plus operator takes precedence over the minus here. The reason for this could be that an interval minus a date is an invalid operation, but the documentation also implies that this is the case (doesn't come out and say it). So, the first operation performed is date + interval. A date plus an interval is a date. Leaving just

date - date

As per the documentation, this results in an integer representing the number of days. However, you multiplied the original interval by 1,440, so this now represented 1,440 times the amount of days it otherwise would have. You're then left with the number of seconds.

It's worth noting that:

When interval calculations return a datetime value, the result must be an actual datetime value or the database returns an error. For example, the next two statements return errors:

The "trick" method will fail, rarely but it will still fail. As ever it's best to do it properly.