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

This is a nice puzzle.

As my main DBMS is Teradata I wrote a solution for it using Analytical functions (needs TD14.10+):

SELECT dt.*,
   -- find the last item in the stack with the same position
   Last_Value(val IGNORE NULLS)
   Over (PARTITION BY pos
         ORDER BY i) AS top_of_stack_val
FROM 
 ( 
   SELECT st.*,
      -- calculate the number of items in the stack
      Sum(CASE WHEN op = 'I' THEN 1 ELSE -1 end) 
      Over (ORDER BY i
            ROWS Unbounded Preceding) AS pos
   FROM stack_trace AS st
 ) AS dt;

This solution works for Oracle, too, but PostgreSQL & SQL Server don't support the IGNORE NULLS option for LAST_VALUE and emulating it is quite complicated, e.g see Itzk Ben-Gan's The Last non NULL Puzzle

Edit: In fact it's not that complex, I forgot Itzik's 2nd solution, the old piggyback trick ;-)

Martin Smith's approach will work for all four DBMSes.