I have two tables - cats and manufacturers. Both have primary 'id' fields. I need to merge them and so want to find out how many have the same id.

I've tried this:
SELECT 'cats.id', 'cats.name'
FROM `cats`
INNER JOIN `manufacturers`
ON 'cats.id'='manufacturers.id'
But that returns an emtry set and I know that cannot be correct.

Any ideas. Thanks