用户表tab_user：
<table>
   <tr>
      <td>id(用户id)</td>
      <td>username(用户名)</td>
      <td>其他字段...</td>
      <td>expenditure(累计消费金额)</td>
   </tr>
   <tr>
      <td>1</td>
      <td>zhangsan</td>
      <td>......</td>
      <td>0</td>
   </tr>
   <tr>
      <td>2</td>
      <td>lisi</td>
      <td>......</td>
      <td>1200</td>
   </tr>
<table>
<br>
折扣表tab_expenditure：
<table>
   <tr>
      <td>id</td>
      <td>min_expenditure(折扣最低消费)</td>
      <td>discount(折扣)</td>
   </tr>
   <tr>
      <td>1</td>
      <td>0</td>
      <td>8折</td>
   </tr>
   <tr>
      <td>2</td>
      <td>1000</td>
      <td>7折</td>
   </tr>
<table>