title: Best MySQL data type to store MD5 hash or NULL
body: <p>I have a PHP application which stores all accounts regardless of whether they are active or not in a single table.  The table has a column called "active" which is either is NULL which means the account is active, or contains a MD5 hash which means the account is inactive.</p>
  
  <p>According to <a href="https://stackoverflow.com/questions/2326584/best-practices-for-efficiently-storing-md5-hashes-in-mysql">Best practices for efficiently storing md5 hashes in mysql</a>, if the column always contains a MD5 hash and never NULL, then BINARY(16) is preferred and CHAR(32) is the next best choice.  Since most of my accounts are active and thus most of the column values will be NULL, am I better off using a different data type such as VARCHAR(32)?</p>