title: Mysql update where records don't exist in table
body: <p>I need some help with an MySQL update.  I have 2 tables as follows:</p>
  
  <pre><code>**Inventory**
    vmvcenter
    vmid
    hostname
  
  **Guest_import**
    vmvcenter
    vmid
    discrepancy
  </code></pre>
  
  <p>I need to put a 'YES' in the guest_import.discrepancy column if the record does not exist in the inventory table.  The key between the tables should be CONCAT(vmvcenter,vmid).</p>