public static function upgrade() {
		$sql = self::$sql;

		$sql::$logger->deactivate();
		try {
			$version = $sql->query(
				"SELECT version FROM meta LIMIT 1")['version'];
		} catch(SQLError $e) {
			$sql::$logger->activate();
			return self::upgrade_tables();
		}
		$sql::$logger->activate();

		if (0 <= version_compare($version, self::TABLE_VERSION))
			return self::$logger->debug(
				"Zapmin: Tables are up-to-date.");

		return self::upgrade_tables($version);
	}