Change the table name to singular, e.g. `PRODUCT_BRAND_RELATION`.

I would normally want a better name for a ‘link relation’ like this, in the same way that person-organisation is called ‘membership’ and person-product is called ‘contribution’. However, I can’t think of anything good in this case; the best I could come up with is ‘product branding’, or perhaps ‘product brand association’. I have a slight preference for `PRODUCT_BRAND_ASSOCIATION`, simply because the language of the [Wikipedia page on branding](http://en.wikipedia.org/wiki/Brand) talks about ‘association’.