title: How to update unique values in SQL using a PostgreSQL sequence?
body: <p>In SQL, how do update a table, setting a column to a different value for each row?</p>
  
  <p>I want to update some rows in a PostgreSQL database, setting one column to a number from a sequence, where that column has a unique constraint. I hoped that I could just use:</p>
  
  <pre><code>update person set unique_number = (select nextval('number_sequence') );
  </code></pre>
  
  <p>but it seems that <em>nextval</em> is only called once, so the update uses the same number for every row, and I get a 'duplicate key violates unique constraint' error. What should I do instead?</p>