Mysql joins multiple tables when query results are out of the question

Because of the company's product needs, now to implement a function, table A put the user's basic information, table B saved a table of the user's behavior, table C and table B the same nature. When asked to query the time can be in B or C table in the count results to sort, so think of join, but there is a problem.

The data structure of 3 sheets is posted first

  CREATE TABLE `A` (  `id` int(11) NOT NULL auto_increment,  `username` varchar(255) default NULL,  PRIMARY KEY  (`id`),) ENGINE=MyISAM AUTO_INCREMENT=1;

  CREATE TABLE `B` (  `id` int(11) NOT NULL auto_increment,  `userid` int(11) default NULL,  `dosomething` varchar(255) default NULL,  PRIMARY KEY  (`id`),  KEY `userid` USING BTREE (`userid`)) ENGINE=MyISAM AUTO_INCREMENT=1;

  CREATE TABLE `C` (  `id` int(11) NOT NULL auto_increment,  `userid` int(11) default NULL,  `dosomething` varchar(255) default NULL,  PRIMARY KEY  (`id`),  KEY `userid` USING BTREE (`userid`)) ENGINE=MyISAM AUTO_INCREMENT=1;

Make your own attempt to find that the query results are different

SELECT u.id, COUNT(s.id) AS sapply, COUNT(uu.id) AS ftotal FROM A AS u RIGHT JOIN B AS s ON u.id = s.userid RIGHT JOIN C AS uu ON u.id = `uu`.`userid` GROUP BY `u`.`id` ORDER BY `ftotal` DESC LIMIT 10

The data is obviously problematic, and the results are separated.

SELECT u.id, COUNT(s.id) AS sapply FROM A AS u RIGHT JOIN B AS s ON u.id = s.userid GROUP BY `u`.`id` ORDER BY `sapply` DESC LIMIT 10

SELECT u.id, COUNT(uu.id) AS ftotal FROM A AS u RIGHT JOIN C AS uu ON u.id = uu.userid GROUP BY `u`.`id` ORDER BY `ftotal ` DESC LIMIT 10

Let's see what the problem is. The first SQL statement is no error, but the result is not.

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