title: how can concatenate two string columns in one column in spark python
body: <p>I want to concatenate two column in dataframe as one column here I want to merge nameFirst and nameLast as column called FULL Name </p>
  
  <pre><code>+---------+---------+--------+
  | playerID|nameFirst|nameLast|
  +---------+---------+--------+
  |aardsda01|    David| Aardsma|
  |aaronha01|     Hank|   Aaron|
  |aaronto01|   Tommie|   Aaron|
  | aasedo01|      Don|    Aase|
  +---------+---------+--------+
  </code></pre>
  
  <p>I'm trying this code :</p>
  
  <pre><code>sqlContext.sql("SELECT playerID,(nameFirst+nameLast) as full_name FROM Master")
  </code></pre>
  
  <p>but it returns </p>
  
  <pre><code>+---------+---------+
  | playerID|full_name|
  +---------+---------+
  |aardsda01|     null|
  |aaronha01|     null|
  |aaronto01|     null|
  | aasedo01|     null|
  </code></pre>
  
  <p>any help please </p>