INSERT INTO wp4t_postmeta (post_id, meta_key, meta_value)
SELECT a.post_id, '_attached_image', b.ID
FROM wp4t_postmeta a
INNER JOIN wp4t_posts b ON a.meta_value WHERE a.meta_key="_content_field_42"
= b.guid

INSERT INTO wp4t_postmeta (post_id, meta_key, meta_value)
SELECT a.post_id, '_attached_image', b.ID
FROM wp4t_postmeta a
INNER JOIN wp4t_posts b ON a.meta_value WHERE a.meta_key="_content_field_42"
= b.guid LIMIT 10;