Take a look at Vadim Tropashko's book SQL Design Patterns for examples of relational division problems and solutions.

Full marks for a good question by the way. It's a bit unfortunate that Relational Algebra and other relational theory ideas aren't as well appreciated and used in the SQL community as they perhaps ought to be. These concepts give you the tools and a vocabulary for solving lots of every day problems.

Unfortunately, as has often been noted, too many database practitioners today don't know the relational model at all. They just know SQL and someone once told them SQL was relational so they assume they must know the relational stuff too.

Not knocking anyone here BTW. Just my thoughts for this "Relational Theory" forum.

Hope this helps.