title: SQL Update with SUM from other fields
body: <p>I have tried many of the solutions available on Stack to solve this problem, but what seems to work for others, doesn't work for me. My situation is very simple and I seem to be unable to simplify the provided solutions down to what I need. </p>
  
  <p>I have two tables:</p>
  
  <p><strong>master</strong></p>
  
  <pre><code>M_ID, Name, total_number
  </code></pre>
  
  <p><strong>things</strong></p>
  
  <pre><code>T_ID, M_ID, thing, number
  </code></pre>
  
  <p>I can query the SUM, however I have been unable to update master.total_number with the sum. I've tried all sorts of solutions and I really think I am over thinking this so I give up! </p>
  
  <p>Here is the query i wish to achieve, knowing full well this doesn't work as is:</p>
  
  <pre><code>UPDATE master
  set master.total_number = (SELECT SUM(things.number) 
                             from things 
                             where M_ID = 1234) 
  where master.M_ID = 1234
  </code></pre>