Mine

CREATE TABLE `participant_info` (
  `ParticipantID` int(10) unsigned NOT NULL default '0',
  `EventYear` int(10) unsigned NOT NULL default '0',
  `ParticipantYears` int(10) unsigned default NULL,
  `ParticipantTypeID` int(10) unsigned default NULL,
  `IsWebsiteAllowed` int(10) unsigned default NULL,
  `ShirtSizeID` int(10) unsigned default NULL,
  `GenderID` int(10) unsigned default NULL,
  PRIMARY KEY  (`ParticipantID`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1

CREATE TABLE `participant_ids` (
  `ParticipantID` int(10) unsigned NOT NULL auto_increment,
  `EventYear` int(10) unsigned NOT NULL default '0',
  `DateCreated` timestamp NOT NULL default '0000-00-00 00:00:00',
  `SessionID` varchar(255) default NULL,
  `ProfileID` int(10) unsigned default NULL,
  `EventID` int(10) unsigned default NULL,
  `TeamID` int(10) unsigned default NULL,
  PRIMARY KEY  (`ParticipantID`),

) ENGINE=MyISAM DEFAULT CHARSET=latin1

explain SELECT count('ParticipantID') FROM participant_ids, participant_info WHERE participant_ids.ParticipantID = participant_info.ParticipantID;

+----+-------------+---------------------------+--------+---------------+---------+---------+-----------------------------------------------------+-------+-------------+
| id | select_type | table                     | type   | possible_keys | key     | key_len | ref                                                 | rows  | Extra       |
+----+-------------+---------------------------+--------+---------------+---------+---------+-----------------------------------------------------+-------+-------------+
|  1 | SIMPLE      | participant_info | index  | PRIMARY       | PRIMARY | 4       | NULL                                                | 43392 | Using index | 
|  1 | SIMPLE      | participant_ids  | eq_ref | PRIMARY       | PRIMARY | 4       | participant_info.ParticipantID |     1 | Using index | 
+----+-------------+---------------------------+--------+---------------+---------+---------+-----------------------------------------------------+-------+-------------+