Aggregate periods using recursive queries

Question

I need to merge overlapping periods (defined by FROM and TO variables) of sequential events (with identifier NUM) for each group (ID) with a \"lookahead buffer\", meaning that i

Answer 1

You should use the RESET WHEN window modifier in your analytic functions (LAG in Teradata 16, or MAX in earlier ones); don't use a recursive query.

Update:

DROP TABLE MY_TABLE;
CREATE VOLATILE TABLE MY_TABLE 
( id          INTEGER
, num         INTEGER
, from_value  INTEGER
, to_value    INTEGER
, lookahead   INTEGER
) ON COMMIT PRESERVE ROWS;

INSERT INTO MY_TABLE VALUES (1, 1, 1,  10, 5);
INSERT INTO MY_TABLE VALUES (1, 2, 13, 20, 5);
INSERT INTO MY_TABLE VALUES (1, 3, 21, 25, 3);
INSERT INTO MY_TABLE VALUES (1, 4, 29, 31, 3);
INSERT INTO MY_TABLE VALUES (1, 5, 40, 50, 3);

INSERT INTO MY_TABLE VALUES (2, 1, 1, 10, 5);
INSERT INTO MY_TABLE VALUES (2, 2, 20, 30, 15);
INSERT INTO MY_TABLE VALUES (2, 3, 40, 41, 5);
INSERT INTO MY_TABLE VALUES (2, 4, 100, 200, 5);
INSERT INTO MY_TABLE VALUES (2, 5, 300, 400, 3);


SELECT  id, first_from_value, to_value
FROM  ( SELECT  id
              , to_value
              , CASE WHEN overlaps_flag = 1
                  THEN  NULL
                  ELSE  COALESCE 
                        ( MIN (from_value) 
                            OVER (PARTITION BY id
                                  ORDER BY from_value
                                  RESET WHEN MAX (overlaps_flag) 
                                               OVER (PARTITION BY id 
                                                     ROWS BETWEEN 
                                                          1 PRECEDING 
                                                      AND 1 PRECEDING) = 0
                                  ROWS BETWEEN UNBOUNDED PRECEDING 
                                           AND 1 PRECEDING)
                        , from_value )
                END AS first_from_value
        FROM  ( SELECT  id, from_value, to_value
                      , MAX (from_value) 
                          OVER (PARTITION BY id 
                                ORDER BY from_value 
                                ROWS BETWEEN 1 FOLLOWING AND 1 FOLLOWING)
                          AS next_from_value
                      , CASE WHEN to_value + lookahead + 1 >= next_from_value
                          THEN 1 ELSE 0 
                        END AS overlaps_flag
                FROM  my_table
              ) AS a
      ) AS a
WHERE first_from_value IS NOT NULL
ORDER BY 1, 2

id  first_from_value    to_value
1   1                   31
1   40                  50
2   1                   10
2   20                  41
2   100                 200
2   300                 400