Question

Pasteid字段是一个有时为空的字段，当它已满时将引用发布表，发布表通过Userid字段连接到用户表。

select `posts`.`id` as `pid`, `posts`.`media` as `pmedia`, `posts`.`created_at` as `pcreated_at`, `posts`.`txt`, `users`.`media` as `umedia`, `users`.`id`, `users`.`name` as `name`, `users`.`username`, `posts`.`options`, `posts`.`likes`, `posts`.`comments`, `posts`.`copyid`, `posts`.`pasteid`, `u`.`name` as `replayname`, `u`.`name` as `replayuser` 
from `posts` 
inner join `usersq` on `users`.`id` = `posts`.`userid` 
inner join `posts` as `p` on `posts`.`pasteid` = `p`.`id` 
inner join `users` as `u` on `u`.`id` = `p`.`userid` 
where (`posts`.`block` = 0 and `users`.`view` = 1) and `users`.`status` not in (1, 3) 
order by `posts`.`created_at` desc 
limit 30

实际上，我们想知道帖子是否已被复制以及从哪个副本复制。上面的代码的问题在于，它仅包含Pasteid字段必须填写的记录，如果为空，则不包含输出

后桌 id txt媒体pasteid用户ID 1 eee 1.jpg 1 2 rrr 1 3 eee 1.jpg 1 2

用户表身份证名 1阿里 2个插孔

如您所见，插孔是从后面复制的。现在，我需要从用户表中获取阿里的名字，并将其放在replayname字段中

Answer 1

只需使用LEFT JOIN而不是INNER JOIN。 LEFT JOIN将返回左表中的所有记录，即使它们在右表中不匹配也是如此。

select `posts`.`id` as `pid`, `posts`.`media` as `pmedia`, `posts`.`created_at` as `pcreated_at`, `posts`.`txt`, `users`.`media` as `umedia`, `users`.`id`, `users`.`name` as `name`, `users`.`username`, `posts`.`options`, `posts`.`likes`, `posts`.`comments`, `posts`.`copyid`, `posts`.`pasteid`, `u`.`name` as `replayname`, `u`.`name` as `replayuser` 
from `posts` 
inner join `usersq` on `users`.`id` = `posts`.`userid` 
left join `posts` as `p` on `posts`.`pasteid` = `p`.`id` 
left join `users` as `u` on `u`.`id` = `p`.`userid` 
where (`posts`.`block` = 0 and `users`.`view` = 1) and `users`.`status` not in (1, 3) 
order by `posts`.`created_at` desc 
limit 30

如何将有时有空值的字段连接到表

1 个答案: