A simple question probably..

I want to create a mysql table to store email addresses of people who don't want to receive emails.

The table should only have to store the email addresses, no other information such as a member id is needed.

I will have to do a SELECT query on the table quite often to see if an email address exists in the table.

For the past few years, I have always used this:

CREATE TABLE `em_opt_out` (
`em_id` mediumint(8) unsigned NOT NULL auto_increment,
`email` varchar(70) NOT NULL default '',
PRIMARY KEY (`em_id`)
) ENGINE=MyISAM;

and then the mysql select query in PHP:
$total = mysql_result(mysql_query("SELECT COUNT(em_id) FROM em_opt_out WHERE email='".quote_smart($email)."'"),0);

Although this works perfectly fine, I have the feeling it is not the most efficient, since I've defined the em_id auto-increment, but never use it.

What would you guys suggest? Is it better to do this for example?

CREATE TABLE `em_opt_out` (
`email` varchar(70) NOT NULL default '',
PRIMARY KEY (`email`)
) ENGINE=MyISAM;

and then the mysql select query in PHP:
$total = mysql_result(mysql_query("SELECT COUNT(email) FROM em_opt_out WHERE email='".quote_smart($email)."'"),0);

Thanks in advance for your help!