title: cassandra add column if not exists
body: <p>I need to insert a new column into an existing column family via a CQL script.</p>
  
  <p>I want to do something like:</p>
  
  <pre><code>alter COLUMNFAMILY rules ADD rule_template text IF NOT EXISTS;
  </code></pre>
  
  <p>How can I achieve this purely in CQL script?</p>