title: Merge with multiple primary keys
body: <p>I have 2 columns that makes a record unique, so I want to merge with multiple keys. How can I do this?</p>
  
  <p>I tried this but it doesnt seem to work:</p>
  
  <pre><code>MERGE INTO TABLE1 AS DST
  USING TABLE2 AS SRC
  ON SRC.pk1 = DST.pk1 
  AND
  SRC.pk2 = DST.pk2 
  
  WHEN NOT MATCHED THEN
  INSERT (pk1, pk2, Description)
  VALUES (SRC.p1, SRC.pk2, SRC.Description)
  ;
  </code></pre>