Date difference to calculate event hours

Hi Tuan,
Rather than manually entering hours into the Event XML custom fields is there a SQL method using the Start / End Date and return the hours?
I tried adding a sql custom field - all i want to do is return the output and show it in the registrants management page in one of the cols and also output it on the certificate.  

Hello

Yes, no need for entering hours manually for each event. It could be dynamic calculated. But it will require customizing code of registrants management layout, not via an SQL field as you are trying to do

Tuan