SQL & PL/SQL

Hi,

in my table I have two columns including hour of start tasks (start_zad) and the second, hour when the tasks where ended (stop_zad).

I try to get information how long taks last. The hour is in format 24HH:MI (eg 23:44) and type of columns is varchar2.

My query is as below and I get wrong result

Thanks for all help

Paul