In reality, we have stumbled in to the world of nested relations – relations can be values. This is a major step for
relational systems. There is much debate on how to proceed. Rather than attack those problems here, we just use
the ALL value as a token representing these sets. Thinking
of the ALL value as the corresponding set defines the semantics of the relational operators (e.g., equals and IN).
The ALL string is for display. A new ALL() function generates the set associated with this value as in the examples
above. ALL() applied to any other value returns NULL.
This design may be eased by SQL3’s support for setvalued variables and domains.