protected function convertToString($type, $expression)
	{
		switch($type)
		{
			case Type::STRING:
			case Type::TEXT:
				return $expression;
			case Type::BIGINT:
			case Type::DECIMAL:
			case Type::FLOAT:
			case Type::INTEGER:
			case Type::SMALLINT:
			case Type::DATE:
			case Type::DATETIME:
			case Type::DATETIMETZ:
			case Type::TIME:
				return 'CAST(' . $expression . ' AS CHAR)';
			default :
				return '\'\'';
		}
	}