title: Add a cell from two or more rows in the same MySql table
body: <p>I have x rows of a column value and "simply" want to add them together and update the highest row number and zero out the other values:</p>
  
  <pre><code>SELECT rowId, addCol FROM table WHERE rowId=1 OR rowId=2
  </code></pre>
  
  <p><strong>rowId</strong> |  <strong>addCol</strong><br />
  1 &nbsp; &nbsp; &nbsp; &nbsp; | 100<br />
  2 &nbsp; &nbsp; &nbsp; &nbsp; | 200<br />
  3 &nbsp; &nbsp; &nbsp; &nbsp; | 67</p>
  
  <p>This is kind of what I want (I know it's completely wrong) but not sure how to express it!</p>
  
  <pre><code>UPDATE table SET addCol3=addCol1+addCol2+addCol3, addCol1=0, addCol2=0
  </code></pre>
  
  <p><strong>rowId</strong> |  <strong>addCol</strong><br />
  1 &nbsp; &nbsp; &nbsp; &nbsp; | 0<br />
  2 &nbsp; &nbsp; &nbsp; &nbsp; | 0<br />
  3 &nbsp; &nbsp; &nbsp; &nbsp; | 367</p>