code: public CharSequence getQueryColumns(String tabalis, SelectColumn selects) {
          if (selects == null) return tabalis == null ? "*" : (tabalis + ".*");
          StringBuilder sb = new StringBuilder();
          for (Attribute attr : this.attributes) {
              if (!selects.test(attr.field())) continue;
              if (sb.length() > 0) sb.append(',');
              sb.append(getSQLColumn(tabalis, attr.field()));
          }
          if (sb.length() == 0) sb.append('*');
          return sb;
      }
docs: 获取查询字段列表
  
  @param tabalis 表别名
  @param selects 过滤字段
  
  @return String