title: pyspark concat multiple columns with coalesce not working
body: <p>The below code throws Column not iterable error.</p>
  
  <p><strong>code:</strong></p>
  
  <pre><code>df = spark.createDataFrame([('abcd', '123')], ['s', 'd'])
  df.select(concat_ws('', coalesce(df.s, "sometext"), coalesce(df.d, "sometext")).alias('sd')).show()
  </code></pre>