private function _prepareColumns(array $columnArray)
	{
		if(count($columnArray) === 0)
		{
			throw new \Exception('The column method of the statement must called before!');
		}
		foreach($columnArray as $column)
		{
			$this->columns .= $column->getColumnName() . ', ';
		}
		$this->columns = rtrim($this->columns);
		$this->columns = rtrim($this->columns, ',');

		return $this;
	}