MySQL incrementing value

Question

Is there a way to make a value increment with every insert if having multiple inserts? (I dont speak of the primary key that autoincrements)

Lets say I have a struct

Answer 1

You will need to ORDER BY id_category and use two user variables so you can track the previous category id -

SET @position := 0;
SET @prev_cat := 0;

INSERT INTO products
SELECT id_product, id_category, name, position
FROM (
    SELECT
        id_product,
        id_category,
        name,
        IF(@prev_cat = id_category, @position := @position + 1, @position := 1) AS position,
        @prev_cat := id_category
    FROM db2.products
    ORDER BY id_category ASC, id_product ASC
) AS tmp;

This will allow you to do all categories in one query instead of category by category.