Finding items from one table that don't exist in another table

Hi,

OK, I hope I will explain this clearly,
I have three tables:
1. wp_posts
2. wp_postmeta
3. companies

wp_posts stores information about the post.
wp_postmeta stores information like custom fields of the post.
companies is a table that gets updated daily with current companies, so if a company is no more our client then they will get deleted from the companies table.

Now, wp_posts and wp_postmeta share post_id and wp_postmeta and companies share company_id, however, wp_postmeta stores company_id as a char while companies stores company_id as an int.

I would like to find all the company_id that exist in wp_postmeta but don't exist in companies and then change their post_status in wp_posts from publish to draft.

I hope that's clear,
Thanks!

So far to find the none existing company_id's I have this:

SELECT p . *
FROM wp_postmeta p
LEFT JOIN companies c ON p.meta_value+0 = c.company_id
WHERE c.company_id IS NULL
AND p.meta_key = 'company_id'
ORDER BY p.meta_value

Now I need to update the status in wp_posts of those companies that are in wp_postmeta table but not in companies table