title: Copy 1 column to another by mysql query
body: <p>I have mysql database with following table...</p>
  
  <pre><code>| id | amount | tax |
  +----+--------+-----+
  |  1 | 500    |     |
  +----+--------+-----+
  |  2 | 100    |     |
  +----+--------+-----+
  </code></pre>
  
  <p>I have to delete amount column but before doing that i want to shift all data of amount row from amount row to tax row. How can it be done using mysql query? Please help.</p>