title: How to convert RDD[Row] to RDD[String]
body: <p>I have a DataFrame called source, a table from mysql</p>
  
  <pre><code>val source = sqlContext.read.jdbc(jdbcUrl, "source", connectionProperties)
  </code></pre>
  
  <p>I have converted it to rdd by </p>
  
  <pre><code>val sourceRdd = source.rdd
  </code></pre>
  
  <p>but its RDD[Row] I need RDD[String]
  to  do transformations like </p>
  
  <pre><code>source.map(rec =&gt; (rec.split(",")(0).toInt, rec)), .subtractByKey(), etc..
  </code></pre>
  
  <p>Thank you</p>