How to deal with missing rows in my stored procedure loop? - mysql

Question

I have this procedure which loops through article values, fetch tag1 and insert it into article_tag table:
DELIMITER $$
CREATE PROCEDURE dt1()
BEGIN
DECLARE maxid INT;
DECLARE x INT;
DECLARE t VARCHAR(30);
DECLARE ntag1 int;
SET maxid = (SELECT MAX(id) FROM `article`);
SET x = (SELECT MIN(id) FROM `article`) ;
WHILE x<= maxid DO
SET t = (SELECT tag1 from `article` WHERE id=x);
SET ntag1 = (SELECT count(*) from `article_tag` WHERE tag=t);
IF ntag1 = 0
THEN
INSERT INTO `article_tag` (tag, slug, frequency) VALUES (t, t, 1);
ELSE
UPDATE `article_tag` SET frequency = frequency + 1 WHERE tag=t;
END IF;
SET x = x + 1;
END WHILE;
END$$
This works fine when there are rows with id in the while loop, but when when there are some missing ids in between (like here)
I get
Query Error: Error: ER_BAD_NULL_ERROR: Column 'tag' cannot be null
I'm wondering what is the idiomatic way to deal with such missing rows?

Answer 1

You can use loop with cursor, so you will only loop through existing records and do not need to check for NULL.
Something like this:
DELIMITER \\
CREATE PROCEDURE dt1()
BEGIN
DECLARE t VARCHAR(30);
DECLARE done INT DEFAULT FALSE;
DECLARE cur1 CURSOR FOR SELECT tag FROM `article` ORDER BY id;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
OPEN cur1;
loop1: LOOP
FETCH cur1 INTO t;
IF done THEN
LEAVE loop1;
END IF;
SELECT COUNT(*) INTO #cnt from `article_tag` WHERE tag = t;
IF #cnt = 0 THEN
INSERT INTO `article_tag` (tag, slug, frequency) VALUES (t, t, 1);
ELSE
UPDATE `article_tag` SET frequency = frequency + 1 WHERE tag = t;
END IF;
END LOOP;
CLOSE cur1;
END
\\
DELIMITER ;
See manual for details:
https://dev.mysql.com/doc/refman/5.7/en/cursors.html
Sure, if possible I'd use INSERT ON DUPLICATE UPDATE as already mentioned.