如何从一列内部连接行？

Question

我今天脑子很慢...
我有一张像这样的表格
WP_PostMeta

 post_id       meta_key       meta_value
      1         magTitle        Title1
      2         magTitle        Title2
      1         magTag          Tag1
      2         magTag          Tag1
      3         magTitle        Title3
      3         magTag          Tag2

我想这样显示此列:

 post_id       magTitle        magTag
      1         Title1          Tag1
      2         Title2          Tag1
      3         Title3          Tag2

我已使用此查询进行了测试

SELECT `meta_value` FROM `wp_postmeta` WHERE `meta_key` = 'magTitle' OR `meta_key` = 'magTag'
INNER JOIN meta_key.magTitle ON meta_key.magTag
ORDER BY post_id ASC

但我错了；

任何曲目都很受欢迎。

Answer 1

你不能真的连接列，你在列上连接表并且你的连接必须在你的where子句中。给定表布局，您可能希望在post_Id上连接，并通过meta_key来约束左右两侧。类似于.

SELECT 
    wppm.`meta_value` as magTitle, 
    wppm2.`meta_value` as magTag 
FROM 
    `wp_postmeta` as `wppm` 
INNER JOIN 
    `wp_postmeta` as `wppm2` 
    ON wppm.post_id = wppm2.post_id 
    AND wppm.meta_key = 'magTitle' 
    AND wppm2.meta_key = 'magTag' 
ORDER BY 
    wppm.post_id ASC