对我有效的是：

DELETE FROM `wp_term_taxonomy` WHERE taxonomy = 'category' AND term_taxonomy_id NOT IN (SELECT term_taxonomy_id FROM `wp_term_relationships`);

DELETE FROM `wp_terms` WHERE term_id NOT IN (SELECT term_id FROM `wp_term_taxonomy`);

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警告1：事先备份数据库可能是一个好主意。
注意事项二：这也会删除非空子类别的空父类别。如果您遇到这种情况，您将不得不在之后更新wp_term_taxonomy表中的parent列。

如果您点击控制面板顶部的“屏幕选项”按钮，您可以选择在WP界面中显示多少个类别，然后使用复选框进行批量删除。

我修改了Dominik的回答，只删除了空的CHILD类别：

DELETE FROM `wp_term_taxonomy` WHERE taxonomy = 'category' AND term_taxonomy_id NOT IN (SELECT term_taxonomy_id FROM `wp_term_relationships`) AND parent <> 0;
DELETE FROM `wp_terms` WHERE term_id NOT IN (SELECT term_id FROM `wp_term_taxonomy`);

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