如何在另一个表中缺少值后清理表？
我尝试将 bbPress 1.0 安装迁移到 WordPress 上的新 bbPress 2.0 插件，但没有成功，这使我的 wp_posts 表变得一团糟。
我通过删除对 post_type 列携带 topic 和 reply 值的所有帖子解决了这个问题，这很好。
但现在我注意到 wp_postmeta 似乎受到许多引用我已删除的帖子的条目的影响。两个表都有 post_id 列。
问题是：我使用哪个 SQL 命令来删除 wp_postmeta 上引用不再位于 wp_posts 上的行的值？我知道这是我应该使用的某种连接，但我不知道如何查找不存在的内容并将其删除。
PS：WordPress 表格不保持引用完整性是怎么回事？我非常确定在删除 wp_posts 上的相关内容时可以自动从 wp_postmeta 中删除某些内容。那好吧…
I was unsuccessful trying to migrate a bbPress 1.0 instalation to the new bbPress 2.0 plugin on my WordPress, which made quite a mess on my wp_posts table.
I solved that by deleting every posts who was carrying the values topic and reply to the post_type column, and that was good.
But now I noticed that the wp_postmeta seems affected by lots of entries refering to posts that I've deleted. Both tables have the post_id column.
The question is: which SQL command I use to delete the values on wp_postmeta referencing lines who aren't anymore on wp_posts? I know it's some kind of join that I should use but I don't see how it would be to look for something who is not there and delete it.
PS: And what's up with WordPress tables not keeping referential integrity? I'm pretty sure it's possible to delete something automatically from wp_postmeta when deleting something related on wp_posts. Oh well…
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手动删除表 a 中与表 b 中不匹配的行的一般方法是：
这可能不是执行批量删除的最有效方法（联接可能会更快），但我通常更喜欢在以下情况下使用子选择这样，它更容易理解，因此降低了 FUBAR 概率（您不想意外删除 table_b 等中的行），
因此在您的情况下，您可能可以这样做：
如果您确定，这些是您想要的行要删除，请将 SELECT * 替换为 删除：
a general way to manually delete rows from table a that have no match in table b is:
this may not be the most efficient way to do a mass delete (joins could probably do it faster), but I usually prefer a subselect in cases like this, it is easier to understand and therefore reduces the FUBAR probability (you don't want to accidentally delete rows in table_b etc)
so in your case you could probably do something like this:
if you are sure, these are the rows you want to delete, replace SELECT * with DELETE:
是的，您想使用外连接。
许多较新的应用程序现在使用实体框架，并且不显式强制执行引用完整性，仅强制执行关系。例如，Atlassian JIRA 不强制执行引用完整性。
Yes, you want to use outer joins.
A lot of newer applications now use entity frameworks and don't explicitly enforce referential integrity, just relationships. Atlassian JIRA, for example, doesn't enforce referential integrity.