[twic]

> The idea to expect exactly one first name of the person with the biggest salary is kinda wrong, since there can be more than one person, and this can obviously not described as a single column per group.

That's true. For this idea to work, there would need to be some framework for picking exactly one row from a group. The simplest thing would be to raise an error if there were multiple candidates, but perhaps there are better ways. I think the operation of picking exactly zero or one things from a group is so common that it's worth making provision for it.