title: How to add column if not exists on PostgreSQL?
body: <p>Question is simple. How to add column <code>x</code> to table <code>y</code>, but only when <code>x</code> column doesn't exist  ? I found only solution <a href="https://stackoverflow.com/questions/9991043/how-can-i-test-if-a-column-exists-in-a-table-using-an-sql-statement">here</a> how to check if column exists.</p>
  
  <pre><code>SELECT column_name 
  FROM information_schema.columns 
  WHERE table_name='x' and column_name='y';
  </code></pre>