Constraints Beyond the ER Model
Functional dependencies:
- e.g., A dept can’t order two distinct parts from the same supplier.
- Can’t express this wrt ternary Contracts relationship.
- Normalization refines ER design by considering FDs.
Inclusion dependencies:
- Special case: Foreign keys (ER model can express these).
- e.g., At least 1 person must report to each manager. (Set of ssn values in Manages rel. set must be subset of supervisor_ssn values in Reports_To rel. set.) Foreign key? Expressible in ER model?
General constraints:
- e.g., Manager’s discretionary budget less than 10% of the combined budget of all departments he or she manages.