Extending Relational Operators
At other end of spectrum:
- Selection allows set comparators and constants and use of select, project inside the formula
- Projection allows arbitrary NF2 algebra expression in addition to simple field names
- Union, difference: recursive definitions
- Cross product: usually just relational.
Example: retrieve title, number of pages of all books by Melville:
- ?[title, ?[pages](copies)](?[author=‘Melville’](B))