title: Add the NOT NULL constraint to a column
body: <p>I'm using PHPMyAdmin and I try to add the NOT NULL constraint to a column of my table.</p>
  
  <p>PHPMyAdmin accepts my following query :</p>
  
  <pre><code>ALTER TABLE `wall` MODIFY `token_message` varchar(40) NOT NULL;
  </code></pre>
  
  <p>But I can still insert empty strings (=NULL), I don't understand why.</p>
  
  <p>PS : If you're going to give me some other queries to add this constraint, note I've have tried these 3 which don't work in my PHPMyAdmin (kind of error : #1064 - You have an error in your SQL syntax; check the manual) :</p>
  
  <pre><code>ALTER TABLE `wall` ALTER COLUMN `token_message` SET NOT NULL;
  ALTER TABLE `wall` ALTER COLUMN `token_message` varchar(40) NOT NULL;
  ALTER TABLE `wall` MODIFY `token_message` CONSTRAINTS token_message_not_null NOT NULL; 
  </code></pre>