I have 3 tables that are unrelated (related that each contains data for a different social network). Each has a datetime field dated- I'm already grouping by hour as you can see below (this one below for linked_in)

SELECT count(*), date_format(dated, '%Y:%m:%d %H') as hour
FROM upd8r_linked_in_accts
WHERE CAST(dated AS DATE) = '".$start_date."'
GROUP BY hour

I would like to know how to do a total across all 3 networks- the tables for the three are

CREATE TABLE IF NOT EXISTS `upd8r_facebook_accts` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `owner_id` varchar(50) NOT NULL,
  `user_id` int(11) NOT NULL,
  `fb_id` bigint(30) NOT NULL,
  `dated` datetime NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 AUTO_INCREMENT=80 ;

CREATE TABLE IF NOT EXISTS `upd8r_linked_in_accts` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `owner_id` varchar(50) NOT NULL,
  `user_id` int(11) NOT NULL,
  `linked_in` varchar(200) NOT NULL,
  `oauth_secret` varchar(100) NOT NULL,
  `first_count` int(11) NOT NULL,
  `second_count` int(11) NOT NULL,
  `dated` datetime NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 AUTO_INCREMENT=200 ;

CREATE TABLE IF NOT EXISTS `upd8r_twitter_accts` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `owner_id` varchar(50) NOT NULL,
  `user_id` int(11) NOT NULL,
  `twitter` varchar(200) NOT NULL,
  `twitter_secret` varchar(100) NOT NULL,
  `dated` datetime NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 AUTO_INCREMENT=9 ;

something like this ?

(SELECT count(*), date_format(dated, '%Y:%m:%d %H') as hour
 FROM upd8r_linked_in_accts 
 WHERE CAST(dated AS DATE) = '".$start_date."')
UNION ALL
(SELECT count(*), date_format(dated, '%Y:%m:%d %H') as hour
 FROM upd8r_facebook_accts
 WHERE CAST(dated AS DATE) = '".$start_date."')
UNION ALL
(SELECT count(*), date_format(dated, '%Y:%m:%d %H') as hour 
 FROM upd8r_twitter_accts 
 WHERE CAST(dated AS DATE) = '".$start_date."')
UNION ALL
GROUP BY hour