title: mysql: how to add? "column1" + "column2" where id_parent is equal
body: <p>i have a table &quot;photos&quot; with a column &quot;relevance&quot; and another one named &quot;trend&quot;.
  i want to (add) &quot;relevance&quot;+&quot;trend&quot; where id_parent equal
  I will set trend=0 after. What i  need is to update the column &quot;relevance&quot; first.</p>
  <pre><code>id_parent   relevance   trend
  1            1000        30
  2            200        10
  3            4000        120
  4            100        110
  </code></pre>
  <p>So far:</p>
  <pre><code>update photos set relevance=relevance+trend where...;
  </code></pre>