我正在尝试优化我的sql子查询。
这是我使用sudo ng serve并选择子查询中的字段的sql查询的旧版本：
UNION经过一番研究后，这是我想出的新方法，它使用了据说更好的SELECT
*
FROM
(
SELECT
t7.*, '7' AS priority
FROM
(
SELECT
`mytable`.`id`,
`mytable`.`title`,
`mytable`.`cover_photo`,
`mytable`.`city_id`,
`mytable`.`city_name`,
`cities`.`name` AS `master_city_name`,
`mytable`.`state_id`,
`mytable`.`state_name`,
`states`.`name` AS `master_state_name`,
`countries`.`id` AS `country_id`,
`countries`.`name` AS `country_name`
FROM
`mytable`
LEFT JOIN `cities` ON `cities`.`id` = `mytable`.`city_id`
LEFT JOIN `states` ON `states`.`id` = `mytable`.`state_id`
LEFT JOIN `countries` ON `countries`.`id` = `mytable`.`country_id`
JOIN `users` ON `users`.`id` = `mytable`.`user_id`
WHERE
`mytable`.`status` = 'published'
AND `mytable`.`city_id` = '161'
) AS t7
UNION
SELECT
t8.*, '8' AS priority
FROM
(
SELECT
`mytable`.`id`,
`mytable`.`title`,
`mytable`.`cover_photo`,
`mytable`.`city_id`,
`mytable`.`city_name`,
`cities`.`name` AS `master_city_name`,
`mytable`.`state_id`,
`mytable`.`state_name`,
`states`.`name` AS `master_state_name`,
`countries`.`id` AS `country_id`,
`countries`.`name` AS `country_name`
FROM
`mytable`
LEFT JOIN `cities` ON `cities`.`id` = `mytable`.`city_id`
LEFT JOIN `states` ON `states`.`id` = `mytable`.`state_id`
LEFT JOIN `countries` ON `countries`.`id` = `mytable`.`country_id`
JOIN `users` ON `users`.`id` = `mytable`.`user_id`
WHERE
`mytable`.`status` = 'published'
AND `mytable`.`state_id` = '12'
) AS t8
UNION
SELECT
t9.*, '9' AS priority
FROM
(
SELECT
`mytable`.`id`,
`mytable`.`title`,
`mytable`.`cover_photo`,
`mytable`.`city_id`,
`mytable`.`city_name`,
`cities`.`name` AS `master_city_name`,
`mytable`.`state_id`,
`mytable`.`state_name`,
`states`.`name` AS `master_state_name`,
`countries`.`id` AS `country_id`,
`countries`.`name` AS `country_name`
FROM
`mytable`
LEFT JOIN `cities` ON `cities`.`id` = `mytable`.`city_id`
LEFT JOIN `states` ON `states`.`id` = `mytable`.`state_id`
LEFT JOIN `countries` ON `countries`.`id` = `mytable`.`country_id`
JOIN `users` ON `users`.`id` = `mytable`.`user_id`
WHERE
`mytable`.`status` = 'published'
AND `mytable`.`country_id` = '64'
) AS t9
) AS t
ORDER BY
`t`.`priority`,
RAND()
LIMIT 25
并在主查询中选择了字段：
LEFT JOIN从性能角度来看，哪一个更可取？