Relational Algebra: Operators
The basic operators (selection, projection, rename, union, difference, Cartesian product) and the derived ones (intersection, joins, division, assignment); union compatibility; result schema and the minimum and maximum number of tuples for each operator under set semantics; set versus bag semantics; the equivalence rules used to rewrite queries, with counterexamples for projection over difference and intersection; empty-relation edge cases and the "at least k" pattern with products only.
