title: set default value to null on postgresql
body: <p>I read <a href="http://www.postgresql.org/docs/current/static/sql-altertable.html" rel="noreferrer">here</a> that I should be able to set the default value of a column like this:</p>
  
  <pre><code>ALTER [ COLUMN ] column SET DEFAULT expression
  </code></pre>
  
  <p>But this:</p>
  
  <pre><code>ALTER address.IsActive SET DEFAULT NULL
  </code></pre>
  
  <p>Gives me this error:</p>
  
  <blockquote>
    <p>ERROR:  syntax error at or near "address" LINE 1:  ALTER address.IsActive SET DEFAULT NULL</p>
  </blockquote>
  
  <p>What have I done wrong? Also, how can I specify multiple columns to have their default value be <code>NULL</code>?</p>