php - 如何计算每行连接中的所有实例？

Question

我有以下方案:

CREATE TABLE IF NOT EXISTS `answers` (
`id` bigint(20) unsigned NOT NULL,
`answer` varchar(200) NOT NULL,
`username` varchar(15) NOT NULL,
`date` datetime NOT NULL,
PRIMARY KEY (`id`,`username`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

INSERT INTO `answers` (`id`, `answer`, `username`, `date`) VALUES
(1, 'gfdsf', 'guy', '2012-12-22 00:00:00'),
(4, 'gfdddsfs', 'maricela', '2012-12-22 00:00:00'),
(4, 'gfddsfs', 'mikha', '2012-12-22 00:00:00'),
(4, 'gfdsfs', 'guy', '2012-12-22 00:00:00');

CREATE TABLE IF NOT EXISTS `questions` (
`id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`asker_username` varchar(15) NOT NULL,
`target_username` varchar(15) NOT NULL,
`question` varchar(200) NOT NULL,
`hide` enum('y','n') NOT NULL DEFAULT 'n',
`date` datetime NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8 AUTO_INCREMENT=5 ;

INSERT INTO `questions` (`id`, `asker_username`, `target_username`, `question`, `date`) VALUES
(1, 'mikha', 'guy', 'testo festo', '2012-12-22 00:00:00'),
(2, 'mikha', 'guy', 'saaaaaaaar', '2012-12-22 00:00:00'),
(3, 'sys.tem', 'every.one', 'test g1', '2012-12-06 00:00:00'),
(4, 'sys.tem', 'every.one', 'test g2', '2012-12-06 00:00:00');

我使用以下查询:

   SELECT        
   questions.id AS questionid,
   COUNT(answers.username) AS count_everyone,
   answers.username  
   FROM questions
   LEFT JOIN answers ON questions.id = answers.id
   GROUP BY questions.id,answers.username

问题出在 COUNT(answers.username。我想计算每个问题的答案，但查询显示计数为 1。例如，问题 ID 4 被回答了 3 次，但是COUNT(answers.username) 将其显示为 1 而不是 3。

这是预期的结果:

         questionid count_everyone  username
               1         1            guy
               2         0            null
               3         0            null
               4         3             guy
               4         3           maricela
               4         3             mikha

这是我实际得到的结果:

         questionid count_everyone  username
               1         1            guy
               2         0            null
               3         0            null
               4         1             guy
               4         1           maricela
               4         1             mikha

谢谢

Answer 1

select q.id, coalesce(j.AnswerCount, 0) as AnswerCount, a.username

from questions q

left outer join

(select id as Qid, count(answer) as AnswerCount
from answers
group by id) j

on q.id = j.Qid

left outer join 
answers a on q.id = a.id