title: Pyspark not displaying anything when calculating skewness on one column
body: <p>I am not able to see results of an aggregate function in pyspark dataframe with display function. I am calculating skewness on one column only at a time</p>
  
  <pre><code>from pyspark.sql import functions as f
  
  val1 = [(20,100),(100,100),(50,100),(0,100),(0,100),(0,0),(0,50),(0,100),(0,20)]
  cols = ['val1','val2']
  tf = spark.createDataFrame(val1, cols)
  display(tf.select(f.skewness(tf['val1']),f.skewness(tf['val2'])))
  </code></pre>
  
  <p>It gives me the following output</p>
  
  <pre><code>DataFrame[skewness(val1): double, skewness(val2): double]
  </code></pre>
  
  <p>I know since its an aggregate function I can use </p>
  
  <pre><code>tf.agg(f.skewness('val1')).show()
  </code></pre>
  
  <p>but I want to use display function, but it's coming empty. Any clues?</p>