Question

Say I have two tables:

articles
categories

There’s a many to many table that connects them.

CREATE TABLE cat2art (
  article_id INT,
  category_id INT
);

For a specific article, I have a ‘new list’ of category id’s, and we need to update the cat2art table with these.

Some categories got removed, some got added and some stayed where they were. What is the most effective way to update this table?

I could naively delete all records with the specified article_id and simply add them again. However, if I were to record a date in that same table that tracks when an article was linked to a category, that information will now be destroyed.

I’m looking for a great pattern that easily solves this issue. This question is specifically for PHP and MySQL, but answers in other languages are also fine provided they are applicable to PHP+MySQL as well.

Answer 1

Other systems support MERGE statement which would do exactly what you want.

However, in MySQL, you would need at least two queries (it cannot delete and insert/update in a single statement):

DELETE
FROM    cat2art
WHERE   art_id = $art_id
        AND cat_id NOT IN ($new_cat_1, $new_cat_2, …);

INSERT
IGNORE
INTO    cat2art
VALUES
($art_id, $new_cat_1),
($art_id, $new_cat_2),
…;