SQL to update custom post taxonomies

Question

I have a custom post "review". It uses the same categories and tags from default posts, so they share "category" and "tag" taxonomies.

But later I changed the "review" register to have their own taxonomies, "review-category", and "review-tag". Of course all tags and categories previously assigned on reviews was unbind from them.

So the question: how can I (if possible) convert all tags and categories only in reviews to the new taxonomies? If I wasn't clear enough:

UPDATE wp_term_taxonomy SET taxonomy='post-tag', parent=0 WHERE taxonomy='tag';

This can convert all "category" to "post-tag", but I need to do it only for reviews. Something like (it doesn't work):

SELECT FROM wp_posts WHERE post_type='review'(
UPDATE wp_term_taxonomy SET taxonomy='post-tag', parent=0 WHERE taxonomy='tag';
)

Answer 1

You will need to join four tables to get categories of custom post type "review"

1. wp_term_relationships
2. wp_posts
3. wp_term_taxonomy
4. wp_terms

by using following query you can get list of categories applied to your custom post type:

SELECT *
FROM wp_term_relationships AS tr
LEFT JOIN wp_posts AS p ON tr.object_id = p.ID
LEFT JOIN wp_term_taxonomy AS tt ON tt.term_taxonomy_id = tr.term_taxonomy_id
LEFT JOIN wp_terms AS t ON t.term_id = tt.term_id
WHERE p.post_type = 'review' AND tt.taxonomy = 'category';

and then update the type of these categories to your new custom category (i.e. "review-category")

UPDATE wp_term_taxonomy SET taxonomy = 'review-category' WHERE term_taxonomy_id IN (1, 71, 72);

change where clause according to your category ids.

For more information visit following WordPress documentation links:
Database_Description
WordPress_Taxonomy

Answer 2

You cant export only categories (or taxonomies generally). Just export all content with Tools/Export and you can delete content except categories from xml try a look wp export all plugin