title: Add primary key to existing table
body: <p>I have an existing table called <code>Persion</code>. In this table I have 5 columns: </p>
  
  <ul>
  <li>persionId</li>
  <li>Pname</li>
  <li>PMid</li>
  <li>Pdescription</li>
  <li>Pamt</li>
  </ul>
  
  <p>When I created this table, I set <code>PersionId</code> and <code>Pname</code> as the <strong>primary key</strong>.</p>
  
  <p>I now want to include one more column in the primary key - PMID. How can I write an <code>ALTER</code> statement to do this? (I already have 1000 records in the table)</p>