title: can't add NOT NULL constraint
body: <p>I have a table called Person in MySQL. There is a column called <code>username</code>. I want to add <code>NOT NULL</code> constraint to this column using the following:</p>
  
  <pre><code>ALTER TABLE Person MODIFY username VARCHAR(32) NOT NULL;
  </code></pre>
  
  <p>But it failed and reported error as:</p>
  
  <pre><code>Error Code: 1265. Data truncated for column 'username' at row 1
  </code></pre>
  
  <p>Why can't I change this column to be NOT NULL?</p>