title: Does merge consider rows in the source table individually or all at once?
body: <p>I would like to know whether merge:</p>
  
  <ol>
  <li>Evaluates each row in the source table separately (i.e. performs the INSERT, UPDATE etc. for that row before moving onto the next row in the source table), or </li>
  <li>Evaluates all rows in the source table are at the 'same time'.</li>
  </ol>
  
  <p>Example: in my source table, there may be two very similar records in the source table. If this is the case, I want the 'second' such record to result in an update to the row in the target table created using the 'first' record in the source table. </p>