title: Why are parentheses used in JOIN USING clauses
body: <p>From the following code example, why do I need to use parentheses () in the JOIN clauses, such as in  JOIN PUBLISHER USING (Publisher_Code)?</p>
  
  <p>I know that not using parentheses would return an error, but I would like to know how the machine reads this statement, per se.</p>
  
  <pre><code>SELECT 
  Book_Title as 'Title',
  Publisher_Name
  FROM BOOK
  JOIN PUBLISHER USING (Publisher_Code)
  JOIN WROTE USING (Book_Code)
  JOIN AUTHOR USING (Author_Num)
  group by  Boo, k_Title, Publisher_Name
  having count(distinct concat(LastName,FirstName)) = 2
  </code></pre>
  
  <p>Thank you!</p>