Hierarchical Data in MySQL

Question

I\'ve got a sort of tree like thing going on in my MySQL database.

I have a database that has categories, and each category has a subcat. I\'m keeping all the catego

Answer 1

I'm assuming you know how to get the ID number and that's not the point of the question. Also, parent_id should also be a FK referencing id, and I would use NULL for the topmost layer, not 0.

If your top-most categories have at most one level of sub-category, you can use this query to get all Toys:

SELECT *
FROM items
WHERE items.category_id IN (SELECT id FROM categories
                            WHERE categories.parent_id = 1
                            OR categories.id = 1);

If your categories can have nested sub-categories, you'll have to use a stored procedure and call it recursively. Pseudocode:

Procedure getItemsInCategory
Input: @category_id integer
Output: items rows
{
    For each item in (SELECT *
                      FROM items
                      WHERE items.category_id = @category_id):
        return the row;

    For each id in (SELECT id 
                    FROM categories
                    WHERE categories.parent_id = @category_id):
        return the rows in getItemsInCategory(id);
}