Constraints in the Model
Domain constraints:
- that values of a column come from a defined domain
Key constraints:
- that there be a key
- that primary key value not be null
"Referential integrity" (in the case where R1 contains a "foreign key" of R2)
- that a foreign key value in R1 always refer to some row in R2 with that key value
Issue: who enforces constraints and how?