Logical Database Design Questions and Answers

Answer: B

Correct answer: False

An enterprise key is unique across the whole database, not merely within one relation. That wider uniqueness is its point: it lets an object be identified without reference to which relation it came from, which matters when relations are merged or when supertypes and subtypes share identifiers.

Answer: B

Correct answer: False

This is the entity integrity rule. A null primary key would leave the row unidentifiable and unreferenceable, so it is forbidden - for every attribute of a composite key as well as for a simple one. A foreign key, by contrast, may be null, which is how optional participation is represented.

Answer: A

Correct answer: no primary key attribute may be null.

A null primary key would identify nothing, so the row could not be distinguished or referenced. The rule applies to every attribute of a composite key as well as to a simple one. It is enforced by the database rather than by the application, which is why a primary key declaration implies not null.

Answer: A

Correct answer: True

A candidate key is by definition a minimal set of attributes that uniquely identifies each row. A relation may have several, and the designer chooses one as the primary key on grounds of stability, simplicity and whether it can ever be null; the others remain candidate keys and are usually given unique indexes.

Answer: B

Correct answer: Two relations are created.

A multivalued attribute cannot be held in a single-valued column, so it is placed in a relation of its own together with the key of the entity it belongs to. That gives two relations: the entity and the attribute’s, joined by the foreign key - and it is exactly what first normal form requires.

Answer: C

Correct answer: Three relations are created.

The two entities each become a relation, and the many-to-many relationship becomes a third, holding the two foreign keys as its composite primary key and any attributes belonging to the association. The relationship cannot be carried by either entity’s relation, because either would need repeating groups.

Answer: C

Correct answer: Four relations are created.

The three participating entities each become a relation, and the associative entity becomes a fourth carrying the three foreign keys as its primary key together with any attributes of the association. The pattern is the same as for a binary many-to-many, which gives three relations for two entities.

Answer: B

Correct answer: False

Well-structured relations are precisely those that avoid anomalies. A relation containing redundancy admits insertion, deletion and update anomalies, and normalisation removes them by splitting the relation until every attribute depends on the key and nothing else.

Answer: A

Correct answer: One relation is created.

A regular entity maps directly to a single relation: its identifier becomes the primary key and its simple attributes become columns. Composite attributes contribute their components, and multivalued attributes are the exception, requiring a relation of their own.