title: PySpark - adding a column to count(*)
body: <p>I Have the below script (i've removed all the column names etc.. to make it easier to see what I am doing at a high level - it was very messy!!)</p>
  
  <p>I need to add a column that is the equivalent of count(*) in SQL. </p>
  
  <p>So if I have grouped user usage by domain I might see the below - where the count is the number of records that match all the prior column conditiosn.</p>
  
  <p>domain.co.uk/       UK User   32433
  domain.co.uk/home   EU User   43464
  etc...</p>
  
  <p>I'm sure it's been asked somewhere on Stackoverflow before, but I've had a good look around and cant find any reference to it!</p>
  
  <pre><code>vpx_cont_filter = vpx_data\
          .coalesce(1000)\
          .join(....)\
          .select(....)\
          .groupBy(....)\
          .agg(
             ....
              )\
          .select(....)
  </code></pre>