title: group by function in neo4j?
body: <pre><code>MATCH(b:Book)
  OPTIONAL MATCH (cu:Customer)-[pu:PURCHASED]-&gt;(:Order)-[]-&gt;(b)
  RETURN pu.ISBN,SUM(pu.qty) AS quantity 
  </code></pre>
  
  <p>I want to return the number of copies of each book sold – unsold books should show as 0 sold copies
  this query does not return the correct answer, I just want to find a way to make a left join from the book node and then group the result by ISBN number. please help </p>
  
  <p><a href="https://i.stack.imgur.com/JlnT2.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/JlnT2.png" alt="enter image description here"></a></p>