Avevi object_id unendosi a term_taxonomy_id che non aveva senso.
Ecco come penso che dovrebbe essere -- avvertenza:non ho mai interrogato un database wp e stavo solo seguendo la documentazione.
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
