Keys in DBMS MCQ

The attributes name, street, and department can repeat for some tuples. But the id attribute has to be unique.So it forms a primary key.

Key is the constraint which specifies uniqueness.

The subset of a set cannot be the same set.Candidate key is a set from a super key which cannot be the whole of the super set

Here the id is the only attribute which can be taken as a key. Other attributes are not uniquely identified.

Super key is the superset of all the keys in a relation.

A relation, say r1, may include among its attributes the primary key of another relation, say r2. This attribute is called a foreign key from r1, referencing r2. The relation r1 is also called the referencing relation of the foreign key dependency, and r2 is called the referenced relation of the foreign key.

Referenced relation

Referencing relation

The primary key has to be referred in the other relation to form a foreign key in that relation.

Street is the only attribute which can occur more than once.

The timeout scheme is particularly easy to implement, and works well if transactions are short and if longwaits are likely to be due to deadlocks.