How do categories and subcategories for WooCommerce are saved in DB?

Question

I have a problem with categories and subcategories in WordPress plugin - WooCommerce. I'm creating a script which would create a categories and subcategories, the problem is that I don't fully understand how all this works in WooCommerce DB structure.

That's what I was able to do:

In "wp_terms":

term_id | name              | slug     | term group
20      | Parent category   | parent   | 0
21      | Children category | children | 0

In "wp_term_taxonomy":

term_taxonomy_id | term_id | taxonomy    | description | parent | count
1                | 20      | product_cat |             | 0      | 0
2                | 21      | product_cat |             | 20     | 0

And that's working, but why the hell this don't:

In "wp_term_taxonomy":

term_taxonomy_id | term_id | taxonomy    | description | parent | count
1                | 20      | product_cat |             | 21     | 0
2                | 21      | product_cat |             | 0      | 0

Answer 1

function getParentCategories() {
    global $wpdb;
    $sql = "SELECT term_id as id, name, slug FROM wp_terms where term_id in (SELECT term_id FROM wp_term_taxonomy where parent = 0 and taxonomy = 'category') order by name asc";
    $parents = $wpdb->get_results( $sql );
    return $parents;
}

function getChildCategories($id) {
    global $wpdb;
    $sql = "SELECT term_id as id, name, slug FROM wp_terms where term_id in (SELECT term_id FROM wp_term_taxonomy where parent = $id and taxonomy = 'category')  order by name asc";
    $children = $wpdb->get_results( $sql );
    return $children;
}