title: SQL - Create column with default value in reference with a timestamp column
body: <p>I'd like to create a column like for example 'post_date_col' in reference with 'pre_date_col'. I would like to field all the rows of the 'post_date_col' with a timestamp default value of 3 months after 'pre_date_col' column ('pre_date_col' is already fielded).</p>
  
  <p>In a "static" way, I was thinking about something like this :</p>
  
  <pre><code> INSERT INTO 'chosen_table'('post_date_col')
   VALUES (UNIX_TIMESTAMP(DATE_ADD(SELECT 'pre_date_col' FROM 'chosen_table', INTERVAL 90 DAY))
  </code></pre>
  
  <p>Is it possible to do it with pure SQL ?</p>
  
  <p>Thanks !</p>