Question

有没有办法在子查询中选择多个列？我想从username表（第3行和第4行）中选择两列dateline和posts，我不想创建两个单独的子查询。或者有没有办法通过加入来做到这一点？

SELECT `topics`.`id` AS `id`, `title`, `unique_views`, `tags`.`name` AS `name`, `bg_color`, `text_color`,
    `username`, `avatar`, `color`,
    (select `username` from `posts` where `topic_id` = `topics`.`id` order by `dateline` desc) as lastpost_username,
    (select `dateline` from `posts` where `topic_id` = `topics`.`id` order by `dateline` desc) as lastpost_dateline,
    (select `vote` from `topic_votes` where `topic_id` = `topics`.`id` and `voter_id` = :voter_id) as user_vote,
    (select count(*) from `topic_votes` where `topic_id` = `topics`.`id` and `vote` = 1) -
    (select count(*) from `topic_votes` where `topic_id` = `topics`.`id` and `vote` = -1) AS vote_sum
FROM `topics`
INNER JOIN `tags` ON `tags`.`id` = topics.`tag_id`
INNER JOIN `users` ON `topics`.`poster_id` = `users`.`id`
INNER JOIN `user_groups` ON `users`.`group_id` = `user_groups`.`id`
INNER JOIN `posts` ON `posts`.`topic_id` = `topics`.`id`
ORDER BY `topics`.`dateline` DESC
LIMIT 50

Answer 1

你可以这样做。你可以加入派生表，但我不能测试：

SELECT `topics`.`id` AS `id`, `title`, `unique_views`, `tags`.`name` AS `name`, `bg_color`, `text_color`,
    `username`, `avatar`, `color`,
    new_table.lastpost_username,
    new_table.lastpost_dateline,
    (select `vote` from `topic_votes` where `topic_id` = `topics`.`id` and `voter_id` = :voter_id) as user_vote,
    (select count(*) from `topic_votes` where `topic_id` = `topics`.`id` and `vote` = 1) -
    (select count(*) from `topic_votes` where `topic_id` = `topics`.`id` and `vote` = -1) AS vote_sum
FROM `topics
LEFT JOIN (
        select `username`,`dateline` FROM `posts`  ORDER BY `dateline` DESC
) AS new_table ON `new_table`.`topic_id` = `topics`.`id`
INNER JOIN `tags` ON `tags`.`id` = topics.`tag_id`
INNER JOIN `users` ON `topics`.`poster_id` = `users`.`id`
INNER JOIN `user_groups` ON `users`.`group_id` = `user_groups`.`id`
INNER JOIN `posts` ON `posts`.`topic_id` = `topics`.`id`
ORDER BY `topics`.`dateline` DESC
LIMIT 50

Answer 2

注意：适用于Postgresql 。
我没有为postgresql找到类似的问题。我希望这对双方都有帮助|

您可以为此使用LATERAL 伪代码

无侧向

SELECT *, T1.ID, S1.name from (
    select *, (select id from mysubquery) as TID from myTable
) AS T1
LEFT JOIN subquery S1 ON (T1.TID=S1.ID)  -- for take name

侧面

select *, TID.id, TID.name  
    from myTable, 
    LATERAL (select id, name from mysubquery) as TID

在子查询中选择多个列

2 个答案: