title: specific mysql update based on group by data
body: <p>This one has me stumpped.</p>
  
  <p>I have 2 tables as so:</p>
  
  <pre><code>METERS
  id | startTime
  
  READINGS
  id | meter_id | readingTime
  </code></pre>
  
  <p>What I want to do is update the <code>meters.startTime</code> to the lowest matching <code>readings.readingTime</code> in 1 sql query.</p>
  
  <p>How do I do this?</p>