get_comments_number of depth-1 (Level 1) (1 post)

I selected only 2 levels of comment for my wordpress site.
How can I display the number of comment of the first level ? (Depth-1)

In the wp_comments table, if comment_parent equals 0, it’s a first-level comment.

function get_num_toplevel_comments() {
    global $wpdb;

    return $wpdb->get_var("
        SELECT COUNT(*) 
        FROM $wpdb->comments
        WHERE comment_parent = 0
    ");
}