protected static function get_tax_rate_classes() {
		global $wpdb;

		$cache_key        = 'tax-rate-classes';
		$tax_rate_classes = wp_cache_get( $cache_key, 'taxes' );

		if ( ! is_array( $tax_rate_classes ) ) {
			$tax_rate_classes = $wpdb->get_results(
				"
				SELECT * FROM {$wpdb->wc_tax_rate_classes} ORDER BY name;
				"
			);
			wp_cache_set( $cache_key, $tax_rate_classes, 'taxes' );
		}

		return $tax_rate_classes;
	}