title: MYSQL won't drop the default value for a timestamp
body: <p>I am trying to drop the default value of a timestamp column.</p>
  
  <p>My DDL looks something like this:</p>
  
  <pre><code>CREATE TABLE bills
  (
      ...
      created_at TIMESTAMP DEFAULT '1971-01-06 00:00:00',
      updated_at TIMESTAMP DEFAULT '1970-01-01 00:00:01',
  );
  </code></pre>
  
  <p>When I try any of the following:</p>
  
  <pre><code>ALTER TABLE bills ALTER COLUMN created_at DROP DEFAULT;
  ALTER TABLE bills ALTER COLUMN created_at SET DEFAULT NULL;
  
  ALTER TABLE bills MODIFY COLUMN created_at TIMESTAMP NULL;
  ALTER TABLE bills MODIFY COLUMN created_at TIMESTAMP NULL DEFAULT NULL;
  
  ALTER TABLE bills CHANGE COLUMN created_at created_at TIMESTAMP NULL;
  ALTER TABLE bills CHANGE COLUMN created_at created_at TIMESTAMP NULL DEFAULT NULL;
  </code></pre>
  
  <p>The MYSQL shell says it has executed correctly.</p>
  
  <p>However, when I check the DDL again, it is clear nothing has changed.</p>
  
  <p>On the other hand, I can alter the default value to a specified value like so:</p>
  
  <pre><code>ALTER TABLE bills ALTER COLUMN created_at SET DEFAULT '1972-02-02 00:00:00';
  </code></pre>
  
  <p>And it updates correctly.</p>
  
  <p>Note that the columns are nullable (as you can see by the DDL) and I have put null data values in the "create_at" column.</p>
  
  <p>The MYSQL version I am using is as v5.7.12</p>
  
  <p>Here is more details about the version I am using:</p>
  
  <pre><code>$ mysql --version
  mysql  Ver 14.14 Distrib 5.7.21, for Linux (x86_64) using  EditLine wrapper
  </code></pre>