Question

首先查询：

SELECT  
    u.id,
    u.first_name,
    u.last_name,
    u.tazkera_id,
    cu.relation_type,
    CASE csp.`shift`
        WHEN '1' THEN 'Morning'
        WHEN '2' THEN 'Afternoon'
        else 'Neither'
    END AS shift
FROM `course_user` as cu
LEFT JOIN `courses` as c ON c.id = cu.`course_id`
LEFT JOIN (
    select users.id,
           users.first_name,
           users.last_name,
           users.tazkera_id
           FROM `users`
) as u ON u.id = cu.`user_id`

left JOIN `course_schedule_prefs`   as csp ON csp.`user_id` = cu.`user_id`

Where cu.relation_type = 1 group by cu.`user_id`;

第二次查询：

SELECT  
    u.id,
    u.first_name,
    u.last_name,
    u.tazkera_id,
    cu.relation_type,
    CASE csp.`shift`
        WHEN '1' THEN 'Morning'
        WHEN '2' THEN 'Afternoon'
        else 'Neither'
    END AS shift
FROM `course_user` as cu
LEFT JOIN `courses` as c ON c.id = cu.`course_id`
LEFT JOIN users as u ON u.id = cu.`user_id`
left JOIN `course_schedule_prefs`   as csp ON csp.`user_id` = cu.`user_id`
Where cu.relation_type = 1 group by cu.`user_id`;

表格：

用户：

CREATE TABLE `users` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `first_name` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
  `last_name` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
  `email` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
  `password` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `is_password_reset` int(11) NOT NULL DEFAULT '0',
  `login_ip` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `login_date` datetime DEFAULT NULL,
  `last_login_ip` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `last_login_date` datetime DEFAULT NULL,
  `is_email_address_verified` int(11) NOT NULL DEFAULT '0',
  `failed_login_attempts` int(11) DEFAULT NULL,
  `last_failed_login_date` datetime DEFAULT NULL,
  `tazkera_id` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `province_code` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `district_code` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `village` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `home_address` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `promote_unique_user_id` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `created_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `updated_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `remember_token` varchar(100) COLLATE utf8_unicode_ci DEFAULT NULL,
  `forgot_token` varchar(100) COLLATE utf8_unicode_ci DEFAULT NULL,
  `email_confirm_token` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `district_other` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `father_name` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `training_provider_id` int(10) unsigned DEFAULT NULL,
  `is_profile_completed` int(11) DEFAULT NULL,
  `active` int(11) DEFAULT '1',
  `picture` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `small_picture` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `last_poll` bigint(20) DEFAULT NULL,
  `last_password_reset` date DEFAULT NULL,
  `presence_status` int(11) DEFAULT NULL,
  `auto_presence` int(11) NOT NULL DEFAULT '0',
  `last_activity` timestamp NULL DEFAULT NULL,
  `trash` tinyint(4) NOT NULL DEFAULT '0',
  `login_approval` tinyint(4) NOT NULL DEFAULT '0',
  `lang` varchar(2) COLLATE utf8_unicode_ci NOT NULL DEFAULT 'en',
  `cover` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `small_cover` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `forum_reputation` int(11) NOT NULL DEFAULT '0',
  `forum_suspension_date` date DEFAULT NULL,
  `forum_status` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `users_email_unique` (`email`),
  KEY `users_training_provider_id_foreign` (`training_provider_id`),
  CONSTRAINT `users_training_provider_id_foreign` FOREIGN KEY (`training_provider_id`) REFERENCES `training_providers` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=1000038 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

course_users：

CREATE TABLE `course_user` (
  `course_id` int(10) unsigned DEFAULT NULL,
  `user_id` int(10) unsigned DEFAULT NULL,
  `relation_type` int(11) DEFAULT NULL,
  `created_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `updated_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  KEY `course_user_course_id_foreign` (`course_id`),
  KEY `course_user_user_id_foreign` (`user_id`),
  CONSTRAINT `course_user_course_id_foreign` FOREIGN KEY (`course_id`) REFERENCES `courses` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `course_user_user_id_foreign` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

课程;

CREATE TABLE `courses` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `curriculum_id` int(10) unsigned DEFAULT NULL,
  `training_center_id` int(10) unsigned DEFAULT NULL,
  `training_coordinator_id` int(10) unsigned DEFAULT NULL,
  `focal_point_id` int(10) unsigned DEFAULT NULL,
  `start_date` date DEFAULT NULL,
  `end_date` date DEFAULT NULL,
  `start_time` time DEFAULT NULL,
  `end_time` time DEFAULT NULL,
  `created_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `updated_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `shift` int(11) DEFAULT NULL,
  `stage_id` int(10) unsigned DEFAULT NULL,
  `stream_id` int(10) unsigned DEFAULT NULL,
  `unit_id` int(10) unsigned DEFAULT NULL,
  `master_trainer_id` int(10) unsigned DEFAULT NULL,
  `status` int(11) DEFAULT NULL,
  `pre_test_generated` int(11) NOT NULL DEFAULT '0',
  `post_test_generated` int(11) NOT NULL DEFAULT '0',
  `conduct_days` int(11) DEFAULT NULL,
  `no` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `evaluation_available` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `courses_curriculum_id_foreign` (`curriculum_id`),
  KEY `courses_training_coordinator_id_foreign` (`training_coordinator_id`),
  KEY `courses_focal_point_id_foreign` (`focal_point_id`),
  KEY `courses_stage_id_foreign` (`stage_id`),
  KEY `courses_stream_id_foreign` (`stream_id`),
  KEY `courses_unit_id_foreign` (`unit_id`),
  KEY `courses_master_trainer_id_foreign` (`master_trainer_id`),
  KEY `courses_training_center_id_foreign` (`training_center_id`),
  CONSTRAINT `courses_curriculum_id_foreign` FOREIGN KEY (`curriculum_id`) REFERENCES `curriculums` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `courses_focal_point_id_foreign` FOREIGN KEY (`focal_point_id`) REFERENCES `users` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `courses_master_trainer_id_foreign` FOREIGN KEY (`master_trainer_id`) REFERENCES `users` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `courses_stage_id_foreign` FOREIGN KEY (`stage_id`) REFERENCES `stages` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `courses_stream_id_foreign` FOREIGN KEY (`stream_id`) REFERENCES `streams` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `courses_training_center_id_foreign` FOREIGN KEY (`training_center_id`) REFERENCES `training_centers` (`id`),
  CONSTRAINT `courses_training_coordinator_id_foreign` FOREIGN KEY (`training_coordinator_id`) REFERENCES `users` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `courses_unit_id_foreign` FOREIGN KEY (`unit_id`) REFERENCES `units` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=139 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

course_schedule_prefs;

CREATE TABLE `course_schedule_prefs` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `cunduct_days` int(11) DEFAULT NULL,
  `training_centers` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
  `shift` int(11) NOT NULL,
  `created_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `updated_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `user_id` int(10) unsigned DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `course_schedule_prefs_user_id_foreign` (`user_id`),
  CONSTRAINT `course_schedule_prefs_user_id_foreign` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=24 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

我的问题是，这个查询中哪一个在性能和准确性方面都很好。为什么？

第一个查询：我直接加入表;选择表的所有列。

第二个查询：我在join中使用了子查询;只选择那些我需要它的元素。

注意：我的主要问题是关于加入

的users表

Answer 1

这里有太多可能性。没有简单的，直接的答案。

使用LEFT JOIN（而不是JOIN），优化工具无法启动子查询，而通常会这样做。

如果有WHERE子句和，它对于过滤行非常有用，那么优化程序希望从过滤最多的表开始。你的WHERE子句闻起来像一面旗帜，通常不是很有用。

如果子查询具有GROUP BY或LIMIT，则可能会显着缩小要使用的行数。（不是你的情况。）这个可能使子查询方法更好。

如果cu有INDEX(relation_type, user_id)，则优化工具可以决定处理所有WHERE和GROUP BY是最好的选择，从而以cu作为第一个＆＃39;开始表

在MySQL的旧版本中，JOIN ( SELECT ... )中的子查询无法获取索引，从而导致该tmp表的低效表扫描。在 new 版本中（我认为从5.6开始），优化器在创建tmp表之后采取额外步骤（这需要花费一些成本）来生成最佳索引（在这种情况下为(id)）在JOINing之前。

更多

需要2个表定义。

你有我建议的索引吗？

InnoDB确实需要PRIMARY KEY - 有关course_user的建议，请参阅many:many。

MySQL与Direct表或表的子字段连接

1 个答案: