Mysql从同一行中的同一行返回多列

Question

假设我的表名“POST”只包含“POST_ID”，我有另一个表名post_meta，它还包含“POST_ID”（作为外键）和另外两个列“meta_key”和“meta_value”

我表的结构是

邮政表：

post_ID
1
2

元表：

meta_id      post_id        meta_key          meta_value 
1              1            suite_size        1000
2              1            suite_number      10 
3              2            suite_size        2000
4              2            suite_number      20  

我想创建一个像这样的表

post_id   suite_size  suite_number
1         1000         10
2         2000         20  

我怎样才能做到这一点 ？

Answer 1

尝试这个：

SELECT pm.post_ID,
       SUM(CASE WHEN pm.meta_key = 'suite_size' THEN pm.meta_value ELSE 0 END) as suite_size,
       SUM(CASE WHEN pm.meta_key = 'suite_number' THEN pm.meta_value ELSE 0 END) as suite_number
FROM post_meta pm 
GROUP BY pm.post_ID;

要么

如果你想要post_meta表中的所有帖子是否不存在，那么使用下面的查询

SELECT p.post_ID,
       SUM(CASE WHEN pm.meta_key = 'suite_size' THEN pm.meta_value ELSE 0 END) as suite_size,
       SUM(CASE WHEN pm.meta_key = 'suite_number' THEN pm.meta_value ELSE 0 END) as suite_number
FROM post p
LEFT JOIN post_meta pm ON p.post_id = pm.post_ID
GROUP BY p.post_ID;

Answer 2

您需要使用pivot技术将行显示为列才能实现此目的

select
p.post_ID,
max(case when pm.meta_key = 'suite_size' then pm.meta_value end) as `suite_size`,
max(case when pm.meta_key = 'suite_number' then pm.meta_value end) as `suite_number`
from post p
join post_meta pm on pm.post_id = p.post_ID
group by p.post_ID