title: set a value to null in cassandra with cql
body: <p>How can I set a value of a row to null in cassandra?
  For example I have a table like this:</p>
  
  <pre><code>CREATE TABLE instruments (
                                                code text,
                                                ric text,
                                                primary key (code)
                                              )
  </code></pre>
  
  <p>if I want an element to have a particular <code>ric</code>:</p>
  
  <pre><code>update instruments set ric='test' where code='code';
  </code></pre>
  
  <p>But what about setting <code>ric</code> to <code>None</code>?</p>