Logical Database Design Questions and Answers

Answer: B

Correct answer: Two relations are created.

A unary many-to-many relationship - a part composed of other parts, an employee supervising employees - needs the entity’s own relation plus an associative relation holding pairs of its key. The associative relation’s key is the two roles taken together, each a foreign key to the same relation.

Answer: B

Correct answer: False

Underlining the primary key is the standard convention for writing a relation, and it is how a reader tells the key from the other attributes. Foreign keys are conventionally marked too, usually by a dashed underline or an italic. The notation carries information rather than decorating it.

Answer: A

Correct answer: True

A null foreign key means the row participates in no relationship, which is exactly how optional participation is expressed. Referential integrity requires only that a foreign key which does hold a value must match an existing primary key - it does not require a value to be present.

Answer: B

Correct answer: Second normal form

Second normal form requires first normal form, which is what the absence of multivalued attributes gives, plus the absence of partial dependencies on part of a composite key. Third normal form would need transitive dependencies removed as well, and nothing here says they have been.

Answer: A

Correct answer: The supertype primary key is assigned to each subtype.

Each subtype becomes a relation whose primary key is the supertype’s primary key, which is also a foreign key referring back to it. This is what makes the subtype row and the supertype row the same entity seen twice, and it is why a subtype instance cannot exist without its supertype instance.

Answer: A

Correct answer: True

The associative relation’s primary key is the two foreign keys taken together, since a pairing is identified by which two rows it joins. Where the same pair may recur - the same student enrolling on the same course twice - a further attribute such as a date must be added to keep the key unique.

Answer: B

Correct answer: False

A weak entity does become a relation. What distinguishes it is the primary key: the partial identifier of the weak entity combined with the primary key of the owner, which is also a foreign key. That composite key is what expresses the existence dependence in the relational model.

Answer: A

Correct answer: data in the form of tables.

The relational model presents all data as relations, which appear as two-dimensional tables of rows and columns, and provides operations that take relations and return relations. Its aims are the opposite of the other options: redundancy is minimised by normalisation and the data is highly organised.

Answer: C

Correct answer: An attribute that serves as the primary key of another relation

A foreign key is how relations are joined: it holds the value of some other relation’s primary key, and referential integrity requires that value either to exist there or to be null. It may be part of its own relation’s primary key too, as in an associative relation.

Answer: A

Correct answer: First normal form

First normal form is defined by every attribute being single-valued, so removing the multivalued attributes achieves it and nothing more. Partial and transitive dependencies may still be present, and removing those is what the higher forms are for.