UPDATE SELECT

update.md

Import values from a related table

UPDATE wp_posts AS posts,
	(SELECT post_id, meta_value FROM wp_postmeta 
		WHERE meta_key = 'wpcf-p-desc') AS meta
	SET posts.post_content = meta.meta_value
WHERE posts.ID = meta.post_id
AND posts.post_content = ''
;

Verification query:

SELECT posts.post_content, meta.meta_value
FROM wp_posts AS posts
INNER JOIN wp_postmeta AS meta
	ON posts.ID = meta.post_id
WHERE meta.meta_key = 'wpcf-p-desc'
#	AND posts.post_content != meta.meta_value