code: protected String createSQLOrderby(Flipper flipper) {
          if (flipper == null || flipper.getSort() == null) return "";
          final String sort = flipper.getSort();
          if (sort.isEmpty() || sort.indexOf(';') >= 0 || sort.indexOf('\n') >= 0) return "";
          String sql = this.sortOrderbySqls.get(sort);
          if (sql != null) return sql;
          final StringBuilder sb = new StringBuilder();
          sb.append(" ORDER BY ");
          if (isNoAlias()) {
              sb.append(sort);
          } else {
              boolean flag = false;
              for (String item : sort.split(",")) {
                  if (item.isEmpty()) continue;
                  String[] sub = item.split("\\s+");
                  if (flag) sb.append(',');
                  if (sub.length < 2 || sub[1].equalsIgnoreCase("ASC")) {
                      sb.append(getSQLColumn("a", sub[0])).append(" ASC");
                  } else {
                      sb.append(getSQLColumn("a", sub[0])).append(" DESC");
                  }
                  flag = true;
              }
          }
          sql = sb.toString();
          this.sortOrderbySqls.put(sort, sql);
          return sql;
      }
docs: 根据Flipper获取ORDER BY的SQL语句，不存在Flipper或sort字段返回空字符串
  
  @param flipper 翻页对象
  
  @return String