@Override
	public String toString() {
		StringBuilder sb = new StringBuilder();
		String nl = System.getProperty("line.separator");
		sb.append(relation).append(nl);

		sb.append("Number of attributes: ").append(attribute.size()).append(nl);
		sb.append("-----------").append(nl);
		for (String a : attribute)
			sb.append(a).append(nl);
		sb.append("-----------").append(nl);
		for (HashMap<String, String> row : rows) {
			sb.append(row).append(nl);
		}

		return sb.toString();
	}