private function _checkJoinArray()
	{
		$defaultJoinArray = array('way' => null, 'tables' => null, 'conditionType' => null, 'condition' => null);
		if($this->joinArray === $defaultJoinArray)
		{
			throw new \Exception('one of the methods: inner(), natural(), left[Outer](), right[Outer] must called before!');
		}
	}