public function addSelect($columns) {
		if(is_array($columns))
			return $this->_addSelect($columns);

		$columns = explode(',', $columns);
		foreach($columns as $columnstr) {
			$columnstr = trim($columnstr);

			preg_match('/(.*?)\s+as\s+([a-z_][a-zA-Z0-9_]*)?$/i', $columnstr, $matches);
			if(isset($matches[2])) {
				$alias = $matches[2];
				$column = $matches[1];
			}
			else
				$alias = $column = $columnstr;

			if(isset($this->columns[$alias]))
				throw new \Exception('Column alias '.$alias.' is already used.');
			$this->columns[$alias] = $column;
		}

		return $this;
	}