I was not sure of the total_duration field type in :

> select phone_number, sum(duration - cast('00:00:00' as time)) as total_duration
> from phone_calls
> group by phone_number
> where ...

Finally, if I want to to be sure I get an integer, I can write :

cast(sum(duration - cast('00:00:00' as time)) as int)

Though, I don't know if this is a good practice :-\.

--
Redlet