title: Why agg() in PySpark is only able to summarize one column at a time?
body: <p>For the below dataframe</p>
  
  <pre><code>df=spark.createDataFrame(data=[('Alice',4.300),('Bob',7.677)],schema=['name','High'])
  </code></pre>
  
  <p>When I try to find min &amp; max I am only getting min value in output.</p>
  
  <pre><code>df.agg({'High':'max','High':'min'}).show()
  </code></pre>
  
  <pre class="lang-none prettyprint-override"><code>+-----------+
  |min(High)  |
  +-----------+
  |    2094900|
  +-----------+
  </code></pre>
  
  <p>Why can't agg() give both max &amp; min like in Pandas?</p>