Final answer:
The key attribute of a relation includes all attributes of the relation in its closure.
Step-by-step explanation:
An attribute A is called the key of relation R if its closure includes all attributes of relation R. In other words, the key attribute determines all other attributes in the relation. This is because the closure of an attribute set contains all possible combinations of attributes that can be formed using the given attribute set. Therefore, if the closure of attribute A includes all attributes of relation R, then attribute A is the key of relation R.
An attribute A is called the key of relation R if it functionally determines all the other attributes of relation R. This means that, for any pair of tuples in the relation, if they agree on the key, they must also agree on all other attributes. Thus, no two tuples can have the same value for the key attribute without being identical across all attributes, ensuring uniqueness within the relation. Moreover, the closure of a key attribute set includes all attributes of the relation, which is another way to assert its sufficiency in identifying tuples uniquely.