title: Select rows where a column is null in Cassandra
body: <p>I have following table in Cassandra (CQL spec 3.3.0 ):-</p>
  
  <pre><code>Users: &lt;a,b,c,d&gt;, Primary key a Int, B: Map&lt;Int,Date&gt;
  </code></pre>
  
  <p>Where Users table has column names as <code>a,b,c,d</code>. Now I want to select all the columns for the rows where column b is <code>null</code>. I did following query:-</p>
  
  <pre><code>select from Users where b = null;
  </code></pre>
  
  <p>but this fails with error = <code>"message="Invalid null value in condition"</code></p>
  
  <ol>
  <li>The answer here:- <a href="https://stackoverflow.com/questions/20981075/how-can-i-search-for-records-that-have-a-null-empty-field-using-cql">null doesn't exists</a> clarifies the semantics
  of null but still its not clear how do I select the columns which
  are not present in a row?</li>
  <li>Here <a href="https://stackoverflow.com/questions/11800309/cql-unable-to-null-check-in-where-clause?rq=1">Nulls in Cassandra</a> an answer claims null to be supported by latest cassandra, but looks like it doesn't work for maps.</li>
  </ol>
  
  <p>Is there any way to solve it? Or should I select all the columns from all the rows and manually check null programmatically, which of course would be very inefficient for a large database.</p>