SQL Challenge/Puzzle: Given a stack trace - How to find the top element at each point in time?

Question

• My real life use-case was to merge nested ranges. I\'ve drew some sketches and then I saw the resemblance between ranges starting and ending to stack PUSH and POP oper

Answer 1

Teradata

select      s.i

           ,first_value (s.val) over 
            (
                partition by    s.depth
                order by        s.i
                reset when      s.op = 'I'
            )                                as top_of_stack_val

from       (select      s.i
                       ,s.val
                       ,s.op

                       ,sum (case s.op when 'I' then 1 else -1 end)   over
                        (
                            order by        s.i
                            rows            unbounded preceding
                        )                                                   as depth

            from        stack_trace s
            )
            s
            
order by    i
;