title: Possible to filter Spark dataframe by ISNUMERIC function?
body: <p>I have a <code>DataFrame</code> for a table in SQL. I want to filter this <code>DataFrame</code> if a value of a certain column is numeric or not.</p>
  <pre><code>val df = sqlContext.sql(&quot;select * from myTable&quot;);
  val filter = df.filter(&quot;ISNUMERIC('col_a')&quot;);
  </code></pre>
  <p>I want filter to be a dataframe of <code>df</code> where the values in <code>col_a</code> are numeric.</p>
  <p>My current solution doesn't work. How can I achieve this?</p>