title: Update table a from table b where (conditions)
body: <p>Evening all,</p>
  
  <p>Actually, it's night.  About 11pm.  My brain is shutting down and I need a bit of help so I can finish and go home :)</p>
  
  <p>I have two tables - table a and table b.
  I need to update a field in table a with the value from a field in table b when two other fields match.  The tables don't have a unique id for each record :(</p>
  
  <p>Basically, I want to do this:</p>
  
  <pre><code>update a
  set importantField = 
  (select b.importantfield
  from b
  where a.matchfield = b.matchfield
  and a.matchfield2 = b.matchfield2
  )
  where a.matchfield = b.matchfield
  and a.matchfield2 = b.matchfield2
  </code></pre>
  
  <p>Or at least... I think that's what I want to do...</p>
  
  <p>Can someone help me out, please?</p>