Je had object_id lid worden van term_taxonomy_id wat nergens op sloeg.
Dit is hoe ik denk dat het zou moeten zijn -- waarschuwing:ik heb nog nooit een wp-database opgevraagd en was gewoon afgegaan op de documentatie.
SELECT
p.ID,
p.post_title,
`post_content`,
`post_excerpt`,
t.name AS product_category,
t.term_id AS product_id,
t.slug AS product_slug,
tt.term_taxonomy_id AS tt_term_taxonomia,
tr.term_taxonomy_id AS tr_term_taxonomia,
MAX(CASE WHEN pm1.meta_key = '_price' then pm1.meta_value ELSE NULL END) as price,
MAX(CASE WHEN pm1.meta_key = '_regular_price' then pm1.meta_value ELSE NULL END) as regular_price,
MAX(CASE WHEN pm1.meta_key = '_sale_price' then pm1.meta_value ELSE NULL END) as sale_price,
MAX(CASE WHEN pm1.meta_key = '_sku' then pm1.meta_value ELSE NULL END) as sku
FROM wp_posts p
LEFT JOIN wp_postmeta pm1 ON pm1.post_id = p.ID
LEFT JOIN wp_term_relationships AS tr ON tr.object_id = p.ID
JOIN wp_term_taxonomy AS tt ON tt.taxonomy = 'product_cat' AND tt.term_taxonomy_id = tr.term_taxonomy_id
JOIN wp_terms AS t ON t.term_id = tt.term_id
WHERE p.post_type in('product', 'product_variation') AND p.post_status = 'publish' AND p.post_content <> ''
GROUP BY p.ID,p.post_title