title: MySQL: Iteratively insert a record if not exists
body: <p>I have a huge table like this,</p>
  
  <pre><code>user_id    item     value
        1      aa         1
        1      bb         0
        1      cc         0
        2      aa         1
        2      bb         1
        2      cc         1
        2     xxx         1
        3      aa         0
        3      bb         0
        3      cc         0
      ...     ...       ...
  </code></pre>
  
  <p>I want to insert a record like this</p>
  
  <pre><code>{user_id}     xxx     0
  </code></pre>
  
  <p>for all the users if they don't have this 'xxx' record. Is it possible to do this in SQL?</p>