title: MySQL TIMESTAMP to default NULL, not CURRENT_TIMESTAMP
body: <p>Using MySQL, I'm trying to make a timestamp column from a date column and a time column. If the date or time column contains a NULL value, MySQL automatically sets the corresponding value in the <code>TIMESTAMP</code> column to the <a href="https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_current-timestamp" rel="noreferrer"><code>CURRENT_TIMESTAMP</code></a>. </p>
  
  <p>Is there a way to make it default to a <code>NULL</code>, instead of the <code>CURRENT_TIMESTAMP</code>?</p>
  
  <p>Something like:</p>
  
  <pre><code>ALTER TABLE customers ADD s_timestamp TIMESTAMP;
  UPDATE customers
      SET s_timestamp = timestamp(s_date,s_time) DEFAULT NULL;
  </code></pre>