这如何返回超过 1 个结果？

Question

此查询如何返回超过 1 行？

SELECT username FROM phpbb_users WHERE user_id = (SELECT (FLOOR(RAND() * (5050 - 50 + 1)) + 50));

表元：

CREATE TABLE `phpbb_users` (
  `user_id` mediumint(8) unsigned NOT NULL AUTO_INCREMENT,
  `user_type` tinyint(2) NOT NULL DEFAULT '0',
  `group_id` mediumint(8) unsigned NOT NULL DEFAULT '3',
  `user_permissions` mediumtext COLLATE utf8_bin NOT NULL,
  `user_perm_from` mediumint(8) unsigned NOT NULL DEFAULT '0',
  `user_ip` varchar(40) COLLATE utf8_bin NOT NULL DEFAULT '',
  `user_regdate` int(11) unsigned NOT NULL DEFAULT '0',
  `username` varchar(255) COLLATE utf8_bin NOT NULL DEFAULT '',
  `username_clean` varchar(255) COLLATE utf8_bin NOT NULL DEFAULT '',
  `user_password` varchar(40) COLLATE utf8_bin NOT NULL DEFAULT '',
  `user_passchg` int(11) unsigned NOT NULL DEFAULT '0',
  `user_pass_convert` tinyint(1) unsigned NOT NULL DEFAULT '0',
  `user_email` varchar(100) COLLATE utf8_bin NOT NULL DEFAULT '',
  `user_email_hash` bigint(20) NOT NULL DEFAULT '0',
  `user_birthday` varchar(10) COLLATE utf8_bin NOT NULL DEFAULT '',
  `user_lastvisit` int(11) unsigned NOT NULL DEFAULT '0',
  `user_lastmark` int(11) unsigned NOT NULL DEFAULT '0',
  `user_lastpost_time` int(11) unsigned NOT NULL DEFAULT '0',
  `user_lastpage` varchar(200) COLLATE utf8_bin NOT NULL DEFAULT '',
  `user_last_confirm_key` varchar(10) COLLATE utf8_bin NOT NULL DEFAULT '',
  `user_last_search` int(11) unsigned NOT NULL DEFAULT '0',
  `user_warnings` tinyint(4) NOT NULL DEFAULT '0',
  `user_last_warning` int(11) unsigned NOT NULL DEFAULT '0',
  `user_login_attempts` tinyint(4) NOT NULL DEFAULT '0',
  `user_inactive_reason` tinyint(2) NOT NULL DEFAULT '0',
  `user_inactive_time` int(11) unsigned NOT NULL DEFAULT '0',
  `user_posts` mediumint(8) unsigned NOT NULL DEFAULT '0',
  `user_lang` varchar(30) COLLATE utf8_bin NOT NULL DEFAULT '',
  `user_timezone` decimal(5,2) NOT NULL DEFAULT '0.00',
  `user_dst` tinyint(1) unsigned NOT NULL DEFAULT '0',
  `user_dateformat` varchar(30) COLLATE utf8_bin NOT NULL DEFAULT 'd M Y H:i',
  `user_style` mediumint(8) unsigned NOT NULL DEFAULT '0',
  `user_rank` mediumint(8) unsigned NOT NULL DEFAULT '0',
  `user_colour` varchar(6) COLLATE utf8_bin NOT NULL DEFAULT '',
  `user_new_privmsg` int(4) NOT NULL DEFAULT '0',
  `user_unread_privmsg` int(4) NOT NULL DEFAULT '0',
  `user_last_privmsg` int(11) unsigned NOT NULL DEFAULT '0',
  `user_message_rules` tinyint(1) unsigned NOT NULL DEFAULT '0',
  `user_full_folder` int(11) NOT NULL DEFAULT '-3',
  `user_emailtime` int(11) unsigned NOT NULL DEFAULT '0',
  `user_topic_show_days` smallint(4) unsigned NOT NULL DEFAULT '0',
  `user_topic_sortby_type` varchar(1) COLLATE utf8_bin NOT NULL DEFAULT 't',
  `user_topic_sortby_dir` varchar(1) COLLATE utf8_bin NOT NULL DEFAULT 'd',
  `user_post_show_days` smallint(4) unsigned NOT NULL DEFAULT '0',
  `user_post_sortby_type` varchar(1) COLLATE utf8_bin NOT NULL DEFAULT 't',
  `user_post_sortby_dir` varchar(1) COLLATE utf8_bin NOT NULL DEFAULT 'a',
  `user_notify` tinyint(1) unsigned NOT NULL DEFAULT '0',
  `user_notify_pm` tinyint(1) unsigned NOT NULL DEFAULT '1',
  `user_notify_type` tinyint(4) NOT NULL DEFAULT '0',
  `user_allow_pm` tinyint(1) unsigned NOT NULL DEFAULT '1',
  `user_allow_viewonline` tinyint(1) unsigned NOT NULL DEFAULT '1',
  `user_allow_viewemail` tinyint(1) unsigned NOT NULL DEFAULT '1',
  `user_allow_massemail` tinyint(1) unsigned NOT NULL DEFAULT '1',
  `user_options` int(11) unsigned NOT NULL DEFAULT '230271',
  `user_avatar` varchar(255) COLLATE utf8_bin NOT NULL DEFAULT '',
  `user_avatar_type` tinyint(2) NOT NULL DEFAULT '0',
  `user_avatar_width` smallint(4) unsigned NOT NULL DEFAULT '0',
  `user_avatar_height` smallint(4) unsigned NOT NULL DEFAULT '0',
  `user_sig` mediumtext COLLATE utf8_bin NOT NULL,
  `user_sig_bbcode_uid` varchar(8) COLLATE utf8_bin NOT NULL DEFAULT '',
  `user_sig_bbcode_bitfield` varchar(255) COLLATE utf8_bin NOT NULL DEFAULT '',
  `user_from` varchar(100) COLLATE utf8_bin NOT NULL DEFAULT '',
  `user_icq` varchar(15) COLLATE utf8_bin NOT NULL DEFAULT '',
  `user_aim` varchar(255) COLLATE utf8_bin NOT NULL DEFAULT '',
  `user_yim` varchar(255) COLLATE utf8_bin NOT NULL DEFAULT '',
  `user_msnm` varchar(255) COLLATE utf8_bin NOT NULL DEFAULT '',
  `user_jabber` varchar(255) COLLATE utf8_bin NOT NULL DEFAULT '',
  `user_website` varchar(200) COLLATE utf8_bin NOT NULL DEFAULT '',
  `user_occ` text COLLATE utf8_bin NOT NULL,
  `user_interests` text COLLATE utf8_bin NOT NULL,
  `user_actkey` varchar(32) COLLATE utf8_bin NOT NULL DEFAULT '',
  `user_newpasswd` varchar(40) COLLATE utf8_bin NOT NULL DEFAULT '',
  `user_form_salt` varchar(32) COLLATE utf8_bin NOT NULL DEFAULT '',
  `user_new` tinyint(1) unsigned NOT NULL DEFAULT '1',
  `user_reminded` tinyint(4) NOT NULL DEFAULT '0',
  `user_reminded_time` int(11) unsigned NOT NULL DEFAULT '0',
  PRIMARY KEY (`user_id`),
  UNIQUE KEY `username_clean` (`username_clean`),
  KEY `user_birthday` (`user_birthday`),
  KEY `user_email_hash` (`user_email_hash`),
  KEY `user_type` (`user_type`)
) ENGINE=InnoDB AUTO_INCREMENT=5056 DEFAULT CHARSET=utf8 COLLATE=utf8_bin

Answer 1

原因是WHERE条件：

WHERE user_id = (SELECT (FLOOR(RAND() * (5050 - 50 + 1)) + 50))

每行评估一次。因此，对于每一行，都会进行一次新的调用RAND()，因此查询可以返回 0、1、2、3 或更多行。查询有可能（尽管非常小）返回表的所有行！

与DELETE语句类似的问题，其中调用RAND()在ORDER BY子句中：MySQL subquery 的问题

如果您希望查询永远不会返回超过 1 行，您可以在查询之前评估随机数，因此RAND()调用一次：

SET @random_number = FLOOR(RAND() * (5050 - 50 + 1)) + 50 ;
SELECT username FROM phpbb_users WHERE user_id = @random_number ;

正如@a1ex07 提到的，您还可以使用派生表：

SELECT u.username 
FROM
    (SELECT FLOOR(RAND() * (5050 - 50 + 1)) + 50 AS random_number) AS r
  JOIN 
    phpbb_users AS u
      ON u.user_id = r.random_number ;

不过作为一个警告，我找不到有关此行为的任何文档，因此它很可能在未来版本中发生变化（仅在 5.1 和 5.5 版本中测试。）版本 5.6、5.7 和 MariaDB（5.1、 5.3+) 可能已经改变了这种行为。