title: Add unique index. SQLite3
body: <p>I need to add a unique field index to an existing table. I made this row:</p>
  
  <pre><code>ALTER TABLE auth_user ADD UNIQUE INDEX (email);
  </code></pre>
  
  <p>The table and the field are already exist. 
  The error is:</p>
  
  <blockquote>
    <p>Query Error: near "UNIQUE": syntax error Unable to execute statement</p>
  </blockquote>
  
  <p>What am I missed? Did it have any specific requirements for SQLite3?</p>