$sql1 = "SELECT questions FROM last_check_date WHERE user_id=? ORDER BY questions DESC LIMIT 1";
$sql2 = "SELECT id FROM questions WHERE add_dt>?";
上面的语句有什么作用?
当我执行 sql1
时,它获取用户的最后检查日期。
然后我执行第二个查询,以获取所有 id 的添加日期>上次检查日期(来自 sql1
)并返回受影响的行数。
我想做的是将这 2 个语句合并为 1 个,并优化查询计数。可能会出现以下问题:
$sql1
中没有用户行:必须选择 sql2
中的所有行并返回受影响的行数。
我不知道它应该是什么样子..提前致谢
更新
SHOW CREATE TABLE last_check_date;
结果是
CREATE TABLE `last_check_date` (
`id` int(11) unsigned NOT NULL,
`user_id` bigint(20) unsigned NOT NULL,
`questions` datetime DEFAULT NULL,
`users` datetime DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
和SHOW CREATE TABLE问题;
CREATE TABLE `questions` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`author_id` bigint(20) unsigned DEFAULT NULL,
`question` text NOT NULL,
`var_a` text NOT NULL,
`var_b` text NOT NULL,
`var_c` text NOT NULL,
`var_d` text NOT NULL,
`var_e` text NOT NULL,
`subject` int(11) unsigned DEFAULT NULL,
`chapter` int(11) unsigned DEFAULT NULL,
`section` int(11) unsigned DEFAULT NULL,
`paragraph` int(11) unsigned DEFAULT NULL,
`rank` tinyint(2) NOT NULL,
`add_dt` datetime NOT NULL,
`answer` varchar(1) NOT NULL,
PRIMARY KEY (`id`),
KEY `fk_chapters-id` (`chapter`),
KEY `fk_paragraphs-id` (`paragraph`),
KEY `fk_subjects-id` (`subject`),
KEY `fk_sections-id` (`section`),
KEY `fk_author-id` (`author_id`),
CONSTRAINT `fk_author-id` FOREIGN KEY (`author_id`) REFERENCES `users` (`id`) ON DELETE SET NULL ON UPDATE CASCADE,
CONSTRAINT `fk_chapters-id` FOREIGN KEY (`chapter`) REFERENCES `chapters` (`id`) ON DELETE SET NULL ON UPDATE CASCADE,
CONSTRAINT `fk_paragraphs-id` FOREIGN KEY (`paragraph`) REFERENCES `paragraphs` (`id`) ON DELETE SET NULL ON UPDATE CASCADE,
CONSTRAINT `fk_sections-id` FOREIGN KEY (`section`) REFERENCES `sections` (`id`) ON DELETE SET NULL ON UPDATE CASCADE,
CONSTRAINT `fk_subjects-id` FOREIGN KEY (`subject`) REFERENCES `subjects` (`id`) ON DELETE SET NULL ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8
最佳答案
$sql = "
SELECT q.id
FROM questions q
LEFT JOIN (
SELECT questions
FROM last_check_date
WHERE user_id=?
ORDER BY questions
DESC LIMIT 1
) l ON q.add_dt > l.questions"
$rs = mysql_query($sql);
$rowcount = mysql_num_rows($rs);
我还不知道 PDO/MYSQLI 的正确语法,请适应您喜欢的驱动程序。
关于php - 优化sql语句以降低MySQL服务器负载,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/10884000/