Sometimes when using the WordPress importer, featured images aren't _quite_ imported but report that they were, throwing off has_post_thumbnail() results. This SQL checks if the "connected featured image" actually exists, and if not, removes the half-there connection.

delete_orphaned_featured_image_meta.sql

DELETE pm
FROM wp_postmeta pm
LEFT JOIN wp_posts wp ON wp.ID = pm.meta_value
WHERE
	pm.meta_key = "_thumbnail_id" AND
	wp.ID IS NULL