build groups by finding specific "first and last records"

• rotcha99

  Mr or Mrs. 500

  Points: 594

  More actions

  January 19, 2022 at 10:01 am

  #3974593

  Hello,

  After receiving great help here, I am coming back for a little follow-up. I need to refine the previous question by adding an additional step which goes down to this:

  • find the first "startSignal" in an ordered table and match it with the first "stopSignal" to form a group
  • there can be multiple round-trips

  I've tried to play around with analytic functions and simple min/max joins, but I failed to assign the actual first stop AFTER a running start...

  create table signals
  (
  id int,
  startSignal int,
  stopSignal int
  )
  
  -- some noise to be ignored
  insert into signals values (1, 0, 1);
  insert into signals values (2, 0, 1);
  insert into signals values (3, 0, 1);
  insert into signals values (4, 0, 0);
  insert into signals values (5, 0, 0);
  
  -- this is an actual START (group 1)
  -- because it is seen before a closing STOP
  insert into signals values (6, 1, 0);
  
  -- these are not STARTs because there was no STOP yet
  insert into signals values (7, 1, 0);
  insert into signals values (8, 1, 0);
  insert into signals values (9, 1, 0);
  
  -- more noise
  -- (still within the valid/running START group)
  insert into signals values (10, 0, 0);
  insert into signals values (11, 0, 0);
  
  -- now's the relevant STOP
  -- I want to assign this to the first start (id 6)
  insert into signals values (12, 0, 1);
  
  -- even more noise
  insert into signals values (13, 0, 1);
  insert into signals values (14, 0, 1);
  insert into signals values (15, 0, 0);
  insert into signals values (16, 0, 0);
  
  -- there could be more "round-trips" (groups)
  -- with diffent noise, but the rules stay the same.
  -- it's always: first START and first STOP after this (running) start form a group
  
  -- (same pattern here for demo)
  
  -- some noise to be ignored
  insert into signals values (21, 0, 1);
  insert into signals values (22, 0, 1);
  insert into signals values (23, 0, 1);
  insert into signals values (24, 0, 0);
  insert into signals values (25, 0, 0);
  
  -- this is an actual START (group 1)
  -- because it is seen before a closing STOP
  insert into signals values (26, 1, 0);
  
  -- these are not STARTs because there was no STOP yet
  insert into signals values (27, 1, 0);
  insert into signals values (28, 1, 0);
  insert into signals values (29, 1, 0);
  
  -- more noise
  -- (still within the valid/running START group)
  insert into signals values (30, 0, 0);
  insert into signals values (31, 0, 0);
  
  -- now's the relevant STOP
  -- I want to assign this to the first start (id 26)
  insert into signals values (32, 0, 1);
  
  -- even more noise
  insert into signals values (33, 0, 1);
  insert into signals values (34, 0, 1);
  insert into signals values (35, 0, 0);
  insert into signals values (36, 0, 0);

  Given the demo table above, I want to find & join:

  • id 6 (start) joined with id 12 (first stop after the current start)
  • id 26 (start) joined with id 32 (first stop after the current start)

  Background: I am trying to analyze some hardware data from a HWInfi64 log 🙂

• Eddie Wuerch

  SSChampion

  Points: 12535

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  January 19, 2022 at 4:45 pm

  #3974756

  This is known as a "Data Islands" problem - you're looking for groups of records in an ordered set.

  You have a identity value on the table, so this gets easy. The trick is to first add a ROW_NUMBER() column to the query and subtract that value from the id value to get a grouping value. To get the first and last records for each set of "Start = 1" rows:

  --Select rows where Start = 1 
  SELECT id, StartSignal,StopSignal 
    FROM Signals 
   WHERE StartSignal = 1;
  
  id          startSignal stopSignal
  ----------- ----------- -----------
  6           1           0
  7           1           0
  8           1           0
  9           1           0
  26          1           0
  27          1           0
  28          1           0
  29          1           0
  
  -- Add a ROW_NUMBER() OVER (ORDER BY id), and also subtract that value from the 
  -- id value to get a grouping value:
  SELECT id, row_number() OVER (ORDER BY (id)) AS RowNum, 
         id - (row_number() OVER (ORDER BY (id))) AS GroupVal, 
         StartSignal,StopSignal 
    FROM Signals 
   WHERE StartSignal = 1
  
  id          RowNum               GroupVal             StartSignal StopSignal
  ----------- -------------------- -------------------- ----------- -----------
  6           1                    5                    1           0
  7           2                    5                    1           0
  8           3                    5                    1           0
  9           4                    5                    1           0
  26          5                    21                   1           0
  27          6                    21                   1           0
  28          7                    21                   1           0
  29          8                    21                   1           0
  
  -- Group by GroupVal and pull MIN() and MAX() ids to get your islands:
  WITH StartOnes AS
  (
  SELECT id, id - (row_number() OVER (ORDER BY (id))) AS GroupVal, StartSignal, StopSignal
    FROM signals
   WHERE StartSignal = 1
  )
    SELECT GroupVal, min(id) AS StartOneBegin, max(id) AS StartOneEnd
      FROM StartOnes
     GROUP BY GroupVal
     ORDER BY 1;
  
  GroupVal             StartOneBegin StartOneEnd
  -------------------- ------------- -----------
  5                    6             9
  21                   26            29
  
  -- Repeat these steps WHERE StopSignal = 1 and you have your groups.
  

  Eddie Wuerch
  MCM: SQL