title: Pyspark: how to extract hour from timestamp
body: <p>I have a table like the following</p>
  
  <pre><code>    df
  
   +------------------------------------+-----------------------+
  |identifier                          |timestamp              |
  +------------------------------------+-----------------------+
  |86311425-0890-40a5-8950-54cbaaa60815|2020-03-18 14:41:55 UTC|
  |38e121a8-f21f-4d10-bb69-26eb045175b5|2020-03-13 15:19:21 UTC|
  |1a69c9b0-283b-4b6d-89ac-66f987280c66|2020-03-16 12:59:51 UTC|
  |c7b5c53f-bf40-498f-8302-4b3329322bc9|2020-03-18 22:05:06 UTC|
  |0d3d807b-9b3a-466e-907c-c22402240730|2020-03-17 18:40:03 UTC|
  +------------------------------------+-----------------------+
  
  tmp.printSchema()
  root
   |-- identifier: string (nullable = true)
   |-- timestamp: string (nullable = true)
  </code></pre>
  
  <p>I would like to have a column that take only the day and the hours from the timestamp.</p>
  
  <p>I am trying the following:</p>
  
  <pre><code>from pyspark.sql.functions import hour
  df = df.withColumn("hour", hour(col("timestamp")))
  </code></pre>
  
  <p>but I get the following</p>
  
  <pre><code>+--------------------+--------------------+----+
  |          identifier|           timestamp|hour|
  +--------------------+--------------------+----+
  |321869c3-71e5-41d...|2020-03-19 03:34:...|null|
  |226b8d50-2c6a-471...|2020-03-19 02:59:...|null|
  |47818b7c-34b5-43c...|2020-03-19 01:41:...|null|
  |f5ca5599-7252-49d...|2020-03-19 04:25:...|null|
  |add2ae24-aa7b-4d3...|2020-03-19 01:50:...|null|
  +--------------------+--------------------+----+
  </code></pre>
  
  <p>while I would like to have</p>
  
  <pre><code>+--------------------+--------------------+-------------------+
  |          identifier|           timestamp|hour               |
  +--------------------+--------------------+-------------------+
  |321869c3-71e5-41d...|2020-03-19 03:00:...|2020-03-19 03:00:00|
  |226b8d50-2c6a-471...|2020-03-19 02:59:...|2020-03-19 02:00:00|
  |47818b7c-34b5-43c...|2020-03-19 01:41:...|2020-03-19 01:00:00|
  |f5ca5599-7252-49d...|2020-03-19 04:25:...|2020-03-19 04:00:00|
  |add2ae24-aa7b-4d3...|2020-03-19 01:50:...|2020-03-19 01:00:00|
  +--------------------+--------------------+-------------------+
  </code></pre>