Udtrækker tid til at blive medlem igen

Du kan bruge lag() og filtrer derefter:

select t.*,
       datediff(start, prev_cancelled) as num_days_since_cancel
from (select t.*,
             lag(cancelled) over (partition by id order by start) as prev_cancelled
      from t
     ) t
where prev_cancelled is not null;

Her er en db<>violin.