a set of mathematical principles that form the basis for manipulating relational table; the eight main funcitons are select, project, join, intersect, union, difference, product, and divide.
domain — the set of relations operations:  - unary: select, project - join: cartesian product, theta-join, natural join - set operations: union, intersection, difference - others: division
theoretical way of manipulating table contents using relational operators (select, project, join, union, intersection, subtract, product, rename)