title: Allow some roles to update a column, and some others if it IS NULL
body: <p>I have a column <code>A</code> (type <code>int</code>) in a table for which data is not available at insert time of the other values. I do not want to split the table, as there is no other real reason to do so. I am implementing privilege separation at the database level.</p>
  <p>Only certain users (who belong to category A) should be able to modify <code>column A</code> at any time. But other users (in category B, not necessarily mutually exclusive to category A) should be able to update the column if its value is not set already, ie. if it is <code>NULL</code>.<br />
  I am using PostgreSQL 9.2.4.</p>
  <p>How can I accomplish this? Triggers? Rules? Something else?</p>