title: GROUP BY Works with Select columns not in GROUP BY
body: <p>why is this GROUP BY still working when the SELECTed columns are neither in the GROUP BY clause, nor aggregate function.
  <a href="https://ucde-rey.s3.amazonaws.com/DSV1015/ChinookDatabaseSchema.png" rel="nofollow noreferrer">DATABASE SCHEMA HERE </a></p>
  <pre><code>SELECT FirstName,
     LastName,
     City,
     Email,
     COUNT(I.CustomerId) AS Invoices
  FROM Customers C INNER JOIN Invoices I
  ON C.CustomerId = I.CustomerId
  GROUP BY C.CustomerId
  </code></pre>