title: MySql Table Insert if not exist otherwise update
body: <pre><code>UPDATE AggregatedData SET datenum="734152.979166667", 
  Timestamp="2010-01-14 23:30:00.000" WHERE datenum="734152.979166667";
  </code></pre>
  
  <p>It works if the <code>datenum</code> exists, but  I want to insert this data as a new row if the <code>datenum</code> does not exist. </p>
  
  <p>UPDATE</p>
  
  <p>the datenum is unique but that's not the primary key</p>