Comment by ModernMech

A result in SQL can contain duplicate items unless you tell it explicitly to deduplicate, so uses multiset/bag semantics. The relational model is built on set semantics, where every item is unique. Just because it can express those operations doesn't mean the idea is baked into the language semantics. e.g. the difference between Haskell and Python + first class functions; you can do functional programming in Python but it's not a functional language.