title: How to create unique key base on three field?
body: <p>I have two foreign keys in table:</p>
  
  <pre><code>userId
  visitorId
  </code></pre>
  
  <p>Also field <code>action</code></p>
  
  <p>I want to add constraints on these three field, that userId + visitorId + action will be unique.</p>
  
  <p>How to do that in Workbench, MYSQL?</p>