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

Although it does require an additional step -

Generic solution for Hive, Teradata, Oracle, SQL Server and PostgreSQL

select      s.i

           ,min (s.val) over
            (
                partition by    s.depth
                               ,s.depth_val_seq
            )                                   as top_of_stack_val            

from       (select      s.i
                       ,s.val
                       ,s.depth
                        
                       ,count (s.val) over 
                        (
                            partition by    s.depth
                            order by        s.i
                            rows            between unbounded preceding and current row
                        )                                                                   as depth_val_seq
                         
            from       (select      s.i
                                   ,s.val
                                    
                                   ,sum (case s.op when 'I' then 1 else -1 end)   over
                                    (
                                        order by        s.i
                                        rows            between unbounded preceding and current row
                                    )                                                                   as depth
             
                        from        stack_trace s
                        )
                        s
            )
            s
 
order by    i
;