title: MySQL SUM of one column, Take only one of the same fields
body: <p>Take a total for one column, but only need to take one row for the duplicate</p>
  
  <pre><code>+---------------------------+--------+
  | orderId     | amount               |
  +---------------------------+--------+
  | 2           |   2000               |
  | 2           |   2000               |
  | 5           |   2000               |
  | 7           |   5000               |
  +---------------------------+--------+
  </code></pre>
  
  <p>I hope to get a total of 9000 instead of 11000</p>
  
  <p>I want the total value of different orderIds, not the total value of the same orderId, so I can't use group by.</p>