title: How to delete columns in pyspark dataframe
body: <pre><code>&gt;&gt;&gt; a
  DataFrame[id: bigint, julian_date: string, user_id: bigint]
  &gt;&gt;&gt; b
  DataFrame[id: bigint, quan_created_money: decimal(10,0), quan_created_cnt: bigint]
  &gt;&gt;&gt; a.join(b, a.id==b.id, 'outer')
  DataFrame[id: bigint, julian_date: string, user_id: bigint, id: bigint, quan_created_money: decimal(10,0), quan_created_cnt: bigint]
  </code></pre>
  
  <p>There are two <code>id: bigint</code> and I want to delete one. How can I do?</p>