private function _prepareColumns(array $columnsArray)
	{
		if(count($columnsArray) === 0)
		{
			throw new \Exception('"column"-method of the statement have to invoked before!');
		}
		$columns = ' ';
		foreach($columnsArray as $column)
		{
			$columns .= $column->getExpression() . ', ';
		}
		$columns       = substr($columns, 0, strlen($columns) - 2);
		$this->columns = $columns . ' ';
	}