function setColumns($columns) {
		$this->columns = array();

		if ($columns === null) {
			return $this;
		}

		if (is_string($columns)) {
			return $this->addColumn($columns);
		}

		foreach ($columns as $alias => &$column) {
			$this->addColumn($column, is_int($alias) ? null : $alias);
		}
		return $this;
	}