Suppose, that I have a following resultset:
SELECT
*
FROM
(
SELECT 1 as `no`, NULL as `sequence`
UNION ALL
SELECT 2, ''
UNION ALL
SELECT 3, '1'
UNION ALL
SELECT 4, '1,2,3,4,5'
UNION ALL
SELECT 5, '2,4,5'
UNION ALL
SELECT 6, '1, 5'
UNION ALL
SELECT 7, '1,3,5'
) as `sub`;
My task was to count sequence breaks / holes for each sequence
listed in below. I've written following stored function:
DELIMITER $$
DROP FUNCTION IF EXISTS `countSequenceBreaks`$$
CREATE FUNCTION `countSequenceBreaks`(`sequence` VARCHAR(1000))
RETURNS INT
DETERMINISTIC
BEGIN
DECLARE `delimiter` CHAR DEFAULT ',';
DECLARE `current`, `last` INT;
DECLARE `result` INT DEFAULT 0;
IF
`sequence` IS NULL
OR
NOT LENGTH(`sequence`)
OR
NOT INSTR(`sequence`, `delimiter`)
THEN RETURN `result`;
END IF;
SET `current` = SUBSTRING_INDEX(`sequence`, `delimiter`, 1);
SET `last` = SUBSTRING_INDEX(`sequence`, `delimiter`, -1);
IF `last` < `current`
THEN
SET `result` = `last`;
SET `last` = `current`;
SET `current` = `result`;
SET `result` = 0;
END IF;
WHILE `current` < `last` DO
IF NOT FIND_IN_SET(`current`, `sequence`)
THEN SET `result` = `result` + 1;
END IF;
SET `current` = `current` + 1;
END WHILE;
RETURN `result`;
END$$
DELIMITER ;
But I'm worried about WHILE
-loop might take too much iterations for different sequence members and cause query slowdown.
Questions:
- Is there any way to improve the stored function?
- If there is a way, then how?
My debug query:
SELECT
`no`, `sequence`, `countSequenceBreaks`(`sequence`)
FROM
(
SELECT 1 as `no`, NULL as `sequence`
UNION ALL
SELECT 2, ''
UNION ALL
SELECT 3, '1'
UNION ALL
SELECT 4, '1,2,3,4,5'
UNION ALL
SELECT 5, '2,4,5'
UNION ALL
SELECT 6, '1, 5'
UNION ALL
SELECT 7, '1,3,5'
) as `sub`;
It's resultset:
no sequence `countSequenceBreaks`(`sequence`)
-----------------------------------------------
1 NULL 0
2 0
3 1 0
4 1,2,3,4,5 0
5 2,4,5 1
6 1,5 3
7 1,3,5 2
Regards.