Wordpress: Assign post's featured images via DB

WP_assign_featured_image

-- Substitute # for site ID:

insert into wp_#_postmeta (post_id,meta_key,meta_value) 
select p.id,'_thumbnail_id',min(a.id)
from wp_#_posts p
join wp_#_posts a on (a.post_type = 'attachment' and a.post_parent=p.id)
where p.post_type = 'post'
and p.id not in (select post_id from wp_#_postmeta where meta_key='_thumbnail_id')
group by p.id;