title: How do you concatenate multiple columns in a DataFrame into a another column when some values are null?
body: <p><code>def column_concat(a,b):
      return concat(a,b)<br>
  searches_df = searches_df.withColumn('unique_id',reduce(column_concat,(searches_df[col] for col in search_parameters)))
  </code></p>
  
  <p>This works except when a column contains a null value, then the whole concatenated string is null. I'd like the a place holder or some character instead in the concatenated string. </p>