title: NOT EXISTS together with SELECT NULL
body: <p>I'm trying to figure out what this SQL query is doing, more specifically in the part that starts after <code>NOT EXISTS</code>:</p>
  
  <pre><code>SELECT
      order_num,
      MIN(order_date)
  FROM
      orders
  WHERE
      order_date &gt;= '01.01.2019'
      AND
      NOT EXISTS
      (
          SELECT
              NULL
          FROM
              result
          WHERE
              unique_id = '201895'
              AND
              result = order_num
      )
  GROUP BY
      order_num
  </code></pre>