title: pyspark dataframe merge multiple json file data in one dataframe
body: <p>I am trying to merge multiple json files data in one dataframe before performing any operation on that dataframe. Lets say I have two files file1.txt , file2.txt which contains data like</p>
  
  <p>file1.txt</p>
  
  <pre><code>{"a":1 , "b":"abc", "c":"abc2", "d":"abc3"}
  </code></pre>
  
  <p>file2.txt</p>
  
  <pre><code>{"a":1 , "b":"abc", "c":"abc2", "d":"abc3"}
  </code></pre>
  
  <p>So I am reading both the files one by one like this</p>
  
  <pre><code>range = ["file1","file2"]
  for r in range:
      df = spark.read.json(r)
  df.groupby("b","c","d").agg(f.sum(df["a"]))
  </code></pre>
  
  <p>But the dataframe is overriding the first dataframe data and only showing the 2nd dataframe data. How Can I concat these dataframes? Thanks in advance!</p>