Question

I have 2 mysql tables and I want that:

1) when I delete an article, the associated tags to be automatically deleted

2) when I add/delete a tag, the “hasTags” field from the articles to be auto updated (0 – for 0 tags, 1 – for 1+ tags).

For 1 – I used a foreign key and it works

ALTER TABLE `articles`
  ADD CONSTRAINT `articles_ibfk_1` FOREIGN KEY (`id`) REFERENCES `tags` (`id`) ON DELETE CASCADE;

I don’t know how to do for the second part. It is possible?

The tables are

articles == id | title | content | hasTags

tags == id | articleId | tagname

Thanks!

Answer 1

For part 2 you can either do this on your application code, in the part where you also add/delete a tag.

Update, I would suggest making hastags a reference counter, that way you avoid the problem @James mentioned.

Or you can create 3 triggers:

DELIMITER $$

CREATE TRIGGER ai_tags_each AFTER INSERT ON tags FOR EACH ROW
BEGIN
  UPDATE articles SET hasTags = COALESCE(hastags,0) + 1 WHERE id = NEW.article_id;
END $$

CREATE TRIGGER ad_tags_each AFTER DELETE ON tags FOR EACH ROW
BEGIN
  UPDATE articles SET hasTags = hasTags - 1 WHERE id = OLD.article_id;
END $$

CREATE TRIGGER au_tags_each AFTER UPDATE ON tags FOR EACH ROW
BEGIN
  UPDATE articles SET hasTags = hasTags - 1 WHERE id = OLD.article_id;
  UPDATE articles SET hasTags = COALESCE(hastags,0) + 1 WHERE id = NEW.article_id;
END $$

DELIMITER ;