select 0 or null in crystal reports record expert

Hello All,

I have to include a condition in my code where i need to mention  " price_owner is 0 or null"

"{db_products.price_OWNER} = 0  works fine "

"{db_products.price_OWNER} = 0 or null  does not "   What is the correct way to write it?

Also, please explain the below isnull and not is null in the below lines mean..

not(isnull({product_accounts.num})

isnull({db_users.num})

Thanks a lot for your inputs