右连接查询返回空值记录

Question

CREATE TABLE IF NOT EXISTS `tweets_comment_tbl` (
  `tweet_comment_id` int(12) NOT NULL AUTO_INCREMENT,
  `tweet_id` int(12) NOT NULL,
  `tweets_comment` text NOT NULL,
  `created` int(12) NOT NULL,
  `changed` int(12) NOT NULL,
  `uid` int(12) NOT NULL,
  `userip` varchar(20) NOT NULL,
  `referer` text NOT NULL,
  `status` int(2) NOT NULL DEFAULT '1',
  PRIMARY KEY (`tweet_comment_id`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 AUTO_INCREMENT=6 ;

--
-- Dumping data for table `tweets_comment_tbl`
--

INSERT INTO `tweets_comment_tbl` (`tweet_comment_id`, `tweet_id`, `tweets_comment`, `created`, `changed`, `uid`, `userip`, `referer`, `status`) VALUES
(1, 1, 'COMMENT USER1', 1319395671, 1319395671, 3, '127.0.0.1', 'http://localhost/drupal_tutorial/', 1),
(2, 2, 'comment admin user', 1319395724, 1319395724, 1, '127.0.0.1', 'http://localhost/drupal_tutorial/node', 1),
(3, 2, 'USER COMMENTING HIS COMMENT', 1319395838, 1319395838, 3, '127.0.0.1', 'http://localhost/drupal_tutorial/', 1),
(4, 2, 'ADMIN COMMENTING FOR HIS COMMENT', 1319395865, 1319395865, 1, '127.0.0.1', 'http://localhost/drupal_tutorial/node', 1),
(5, 2, 'dddCOMMENT USER1: ADMIN DOING COMMENT FOR STATUS UPDATE1', 1319395905, 1319395905, 1, '127.0.0.1', 'http://localhost/drupal_tutorial/node', 1);

my second table

CREATE TABLE IF NOT EXISTS `tweets_tbl` (
  `tweet_id` int(11) NOT NULL AUTO_INCREMENT,
  `tweets` text NOT NULL,
  `created` int(12) NOT NULL,
  `changed` int(12) NOT NULL,
  `uid` int(12) NOT NULL,
  `userip` varchar(20) NOT NULL,
  `referer` text NOT NULL,
  `status` int(2) NOT NULL DEFAULT '1',
  PRIMARY KEY (`tweet_id`),
  KEY `tweet_id` (`tweet_id`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 AUTO_INCREMENT=3 ;

--
-- Dumping data for table `tweets_tbl`
--

INSERT INTO `tweets_tbl` (`tweet_id`, `tweets`, `created`, `changed`, `uid`, `userip`, `referer`, `status`) VALUES
(1, 'STATUS UPDATE 1', 1319395633, 1319395633, 1, '127.0.0.1', 'http://localhost/drupal_tutorial/node', 1),
(2, 'Status update user1', 1319395696, 1319395696, 3, '127.0.0.1', 'http://localhost/drupal_tutorial/node', 1);

Trying join query

SELECT ut.picture as picture, tct.tweet_id as tweet_id, tct.tweets_comment as tweets_comment, tct.changed
FROM tweets_comment_tbl tct
RIGHT JOIN users as ut ON tct.uid=ut.uid AND tct.tweet_id=2 AND tct.status = 1
order by tct.created desc

return records for above query

picture         tweet_id    tweets_comment  changed
                  1         COMMENT USER1   1319395671
                NULL    NULL        NULL
picture-1.png   NULL    NULL        NULL

actually what i expected is

picture         tweet_id    tweets_comment  changed
                  1         COMMENT USER1   1319395671

Why query has been returning NULL records, i am not sure where i made mistake in join query.

Answer 1

The query returns all rows from users and joins tweets_comment_tbl based on condition in ON . If no records in tweets_comment_tbl matches userid, the record from users still included into recordset. You probably need INNER JOIN if you want to see just users with comments.

Side note :
I think it's almost always possible to avoid RIGHT JOIN ; queries with LEFT JOIN are much easier to read and understand.