Delete duplicate WordPress posts (where duplication is determined by a post meta value) and subsequently orphaned post meta

delete_duplicate_posts_by_meta_value.sql

DELETE FROM wp_posts WHERE ID IN (
	SELECT post_id
	FROM wp_postmeta
		INNER JOIN (
			SELECT meta_value
			FROM wp_postmeta
			WHERE meta_key = 'meta_key_name'
			GROUP BY meta_value
			HAVING COUNT( meta_value ) > 1) dup
		ON wp_postmeta.meta_value = dup.meta_value
	WHERE post_id NOT IN (
		SELECT 
			MAX(post_id) AS save_this_post_id
		FROM
			wp_postmeta
		WHERE meta_key = 'meta_key_name'
		GROUP BY meta_value
		HAVING COUNT(meta_value) > 1
	)
);

DELETE pm
FROM wp_postmeta pm
LEFT JOIN wp_posts wp ON wp.ID = pm.post_id
WHERE wp.ID IS NULL;