title: ALTER TABLE, set null in not null column, PostgreSQL 9.1
body: <p>I have a table with not null column, How to set a null value in this column as default?</p>
  
  <p>I mean, I want to do something like this:</p>
  
  <pre><code>postgres=# ALTER TABLE person ALTER COLUMN phone SET NULL;
  </code></pre>
  
  <p>but it shows:</p>
  
  <pre><code>postgres=# ALTER TABLE person ALTER COLUMN phone SET NULL;
  ERROR:  syntax error at or near "NULL"
  LINE 1: ALTER TABLE person ALTER COLUMN phone SET NULL;
  </code></pre>