- Uniqueness of a relation's key constraint: Since a relation is a set does not have duplicates then no two tuples are equal. The minimal set of attributes whose value identify uniquely each tuple are called the key attribute set. One key set is designated as the primary key of the relation.
- Entity Integration Constraint (for each relation): no primary key value can have a null in any of the key attribute set.
- Referential Integrity Constraint (between two "joined" relations): is an integrity constraint applied to a relation asserting that the value of a set of attributes in R1 must be equal to R2. When this is broken, we have a dangling reference! The participating set of attributes of relation R1 is called the foreign key.
- Not all possible constraints can be captured directly by the relational model!
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