MySQL join with Direct table or subfields of table - mysql

Question

First query:
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`;
Second Query:
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`;
Tables :
users:
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
courses;
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
My question is , which one of this query is good in term of performance and accuracy. Why ?
first query: I join directly with table; select all columns of table.
Second query: I used subquery inside of join; select only those element which I need it.
Note : My main question is about users table which is in join

Answer 1

There are too many possibilities here. There is no simple, 'straight', answer.
With LEFT JOIN (instead of JOIN), the Optimizer cannot start with the subquery, which it would usually like to do.
If there is a WHERE clause and it is useful for filtering rows, the Optimizer would like to start with the table that is filtered the most. Your WHERE clause smells like a flag, which is usually not very useful.
If a subquery has a GROUP BY or LIMIT, then it may significantly shrink the number of rows to work with. (Not in your case.) This might make the subquery approach better.
If cu has INDEX(relation_type, user_id), the Optimizer may decide that handling all of the WHERE and GROUP BY leads to the best bet, thereby starting with cu as the 'first' table.
In old versions of MySQL, subqueries in JOIN ( SELECT ... ) had no way of getting an index, thereby leading to inefficient table scans of that tmp table. In new versions (starting with 5.6, I think), the Optimizer takes the extra step (which costs something) to generate the optimal index ((id) in this case) after creating the tmp table and before JOINing it.
More
2 more table definitions needed.
Do you have the indexes I suggested?
InnoDB really needs a PRIMARY KEY - See many:many for advice on course_user.