if statement mySQL

I am trying to figure out how to do an if statement that will essentially calculate the stock of an item based on the dates in the tp_rental table. i was hoping to do something like

if Date_Due has an entry but Date_Returned does not then set stock -1 if Date_Due has an entry and Date_returned has an entry entered then set stock +1 See below for my attempt, not sure if i need a join here?

update title_platform
set Stock = case
    when tp_rental.Date_Returned is not null then stock -1
    else stock +1
end

Not sure if i am going about this the right way, any help would be greatly appreciated.

Thank you