title: Enforce a foreign-key constraint to columns of same table
body: <p>How to enforce a constraint of foreign key on columns of same table in SQL while entering values in the following table:</p>
  
  <p><strong>employee</strong>:  </p>
  
  <ul>
  <li>empid number,  </li>
  <li>manager number (must be an existing employee)</li>
  </ul>