docs: protected String insert(String where) {
  		String str = this.sql.toLowerCase();
  		if (str.indexOf(" where ") != -1) {
  			throw new IllegalArgumentException("语句不能包含where关键字.");
  		}
  		int index = str.indexOf("order by ");
  		if (index == -1) {
  			throw new RuntimeException("找不到order by语句.");
  		}
  		return sql.substring(0, index) + where + sql.substring(index);
  	}