convert sum (interval) to seconds

How can I convert sum(interval) to seconds?

select sum(interval_field) from tablename where condition=condition;

ID | LOGIN | INTERVAL
--------------------------
1 | JOHN | 00:00:18
2 | JOHN | 00:45:10

If I say,
select sum(interval) from tablename where login='john';
it will give me 00:45:28 (That's 45 mins, and 28 secs).
How can I convert that to seconds? Can I possibly do it
in just one query? Thank you