title: Selecting map key as column in dataframe in spark
body: <p>I have a dataframe from cassandrasql and I have a column which is a map in dataframe
  like</p>
  
  <pre><code>scala&gt; df.printSchema
  root
   |-- client: map (nullable = true)
   |    |-- key: string
   |    |-- value: string (valueContainsNull = true)
  </code></pre>
  
  <p>I need to select some columns from df as well a particular key from a map as column in df, instead of complete map</p>
  
  <p>Let say I have a map of 
  key1 -> value1
  key2 -> value2
  ....</p>
  
  <p>I need to select only key1 from map in the dataframe to be a column in my new dataframe. How can I do that</p>
  
  <p>Also I am using cassandrasqlcontext.sql to get the dataframe.</p>