Final answer:
Key constraints in a relational database ensure that each tuple in a table has a unique value for its key attributes, maintaining data integrity and enabling consistent data retrieval.
Step-by-step explanation:
The question appears to be asking about the key constraints in a database relation, specifically pertaining to the components A and B where A is a key attribute and B is a nonkey attribute. Key constraints refer to rules that enforce uniqueness for the key attributes in a relation. In relational database terms, a key constraint ensures that every tuple (row) in a table has a unique value or combination of values in the key columns (attributes). For example, if attribute A is a key, then in any instances of relation R such as R1 and R2, each tuple must have a unique value for A. This uniqueness constraint is fundamental to the relational database model and is what allows for the consistent retrieval and manipulation of data.