WooCommerce: assign a category to all products

Something like that should do the trick. Backup before doing it !

UPDATE wp_term_relationships AS tr
JOIN wp_term_taxonomy AS tt ON tr.term_taxonomy_id = tt.term_taxonomy_id
JOIN wp_terms AS t ON tt.term_id = t.term_id
JOIN wp_posts AS p ON tr.object_id = p.ID
SET tt.term_taxonomy_id = {your_category_id}
WHERE tt.taxonomy = 'product_cat'
  AND p.post_type = 'product';

Replace wp_ with your prefix if its different. Replace {your_category_id} with the ID of the category you aim.

Another way is using a function

function assign_all_products_to_category() {
    $category_id = 123; // Replace with the actual ID of your target product category

    // Get all product IDs
    $args = array(
        'post_type' => 'product',
        'posts_per_page' => -1,
    );
    $products = get_posts( $args );

    if ( $products ) {
        foreach ( $products as $product ) {
            $product_id = $product->ID;
            wp_set_object_terms( $product_id, $category_id, 'product_cat', true );
        }
        echo 'All products have been assigned to the category successfully.';
    } else {
        echo 'No products found.';
    }
}

Execute the function as you see fit.

For example run it as url

function check_custom_param() {
    if ( isset( $_GET['run_function'] ) && $_GET['run_function'] === 'assign_products' ) {
        assign_all_products_to_category();
        exit; // Terminate script execution after running the function
    }
}
add_action( 'template_redirect', 'check_custom_param' );

Your url should look like: http://example.com/?run_function=assign_products