在带有CASES的连接表上使用Datediff（）

I'm trying to determine the best way to calculate a datediff (in minutes). I'm trying to determine the time lapsed between two stages in a registration process (stage 8 and stage 10), but I only need this for anyone who's ever been in stage 10. There are two tables: Registration and Registrationstagehistory which are linked by the registration id number (rid)I'm trying to determine the best way to calcula