title: How to deal with null Primary Key in Mysql?
body: <p>I know sql doesn't allow null values in pk.</p>
  <p>but assume we have some nulls in pk, what we can do about it in practice?</p>
  <p>my thoughts</p>
  <ol>
  <li>check other tables to see if we can recover nulls in pk</li>
  <li>ignore null values, as they are not needed to be unique</li>
  </ol>
  <p>please correct me if different thoughts and what else we can do?</p>
  <p>let's generalise the question a bit:</p>
  <p>forget about pk, imagine we have a unique column with some nulls, what do you do about nulls? (we can treat this column as pk even if it is not declared as pk)</p>