Keynote Speakers

WordPress database error: [Column 'post_title' in group statement is ambiguous]
SELECT COUNT( DISTINCT `t`.`ID` ) FROM `wp_posts` AS `t` LEFT JOIN `wp_podsrel` AS `rel_keynote_events` ON `rel_keynote_events`.`field_id` = 1432 AND `rel_keynote_events`.`item_id` = `t`.`ID` LEFT JOIN `wp_posts` AS `keynote_events` ON `keynote_events`.`ID` = `rel_keynote_events`.`related_item_id` LEFT JOIN `wp_postmeta` AS `keynote_events_ID` ON `keynote_events_ID`.`meta_key` = 'ID' AND `keynote_events_ID`.`post_id` = `keynote_events`.`ID` LEFT JOIN `wp_postmeta` AS `last_name` ON `last_name`.`meta_key` = 'last_name' AND `last_name`.`post_id` = `t`.`ID` WHERE ( ( `keynote_events`.`ID` IS NOT NULL ) AND ( `t`.`post_type` = 'participants' ) AND ( `t`.`post_status` IN ( 'publish' ) ) ) GROUP BY post_title