title: SQL merge not matched by target vs not matched by source
body: <p>What is the difference between <code>NOT MATCHED BY SOURCE</code>  vs <code>NOT MATCHED BY TARGET</code>?</p>
  <p><strong>For example:</strong></p>
  <p>Does <code>WHEN NOT MATCHED BY SOURCE</code> mean that the records exist in the target but not in the source? - so we can <strong>delete</strong> them ?</p>
  <p>and <code>WHEN NOT MATCHED BY TARGET</code> - mean the records exists in the source but not in the target? so we can <strong>insert</strong> them?</p>