> is not obvious at all without specifying requirements _first_. If there is a requirement that an employee skill depends on the language (ie. an employee can type in a particular language only) - the design with a single 3 attributes table is the right one.

In the "Baseline" section I do exactly that: specify requirements first. Nowhere does it say anything about skill depending on the language, so I assumed that it implies that they are independent.

"Suppose that we’re building a system that lets people find sports instructors. Each instructor has a number of skills they can teach: yoga, weightlifting, swimming, etc. Also, instructors can speak one or more languages: English, French, Italian, etc."

Not sure how to improve this wording so that it was more clear that skills and languages are independent.

> Multivalued attributes discussion is missing several important aspects for example referential integrity. It is not at all obvious what should happen when a language is deleted from languages table.

I'm not sure what you mean here because multivalued attributes as defined in 1977 preserve referential integrity. But array-typed values do not, you're right.

This is a very important consideration, thank you! I did not realize that (because I don't really think that classical enforced FKs are that crucial in modern practice.) But from a theory point of view having a list of IDs is not enforced, yes.