Logical Database Design Questions and Answers

Q: A relation has which of the following properties?

Correct answer: Each relation has a unique name.Unique naming is a genuine property. The other three are each stated backwards: every row must be unique, no two attributes within a relation may share a name, and the order of columns is not significant - a relation is a set of tuples over named attributes rather than an ordered table.

Q: A composite attribute does not get mapped into a relation.

Correct answer: TrueThe composite attribute itself becomes no column. What is mapped are its simple components - street, town and postcode rather than a single address - each becoming an attribute of the relation. The composite exists in the conceptual model as a grouping and disappears in the mapping.

Q: An enterprise key is a primary key whose value is unique for a given relation.

Correct answer: FalseAn 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.

Q: A primary key may be null.

Correct answer: FalseThis 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.

Q: The entity integrity rule states that:

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.

Q: A candidate key must uniquely identify each row.

Correct answer: TrueA 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.

Q: When mapping a multivalued attribute into a relation which of the following is true?

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.

Q: When mapping a binary many-to-many relationship into a relation which of the following is true?

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.

Q: When mapping a ternary relationship with an associative entity into a relation which of the following is true?

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.

21.

A transitive dependency is which of the following?

A.A functional dependency between two or more key attributes.
B.A functional dependency between two or more nonkey attributes.
C.A relation that is in first normal form.
D.A relation that is in second normal form.

Answer: B

Correct answer: A functional dependency between two or more nonkey attributes.

A transitive dependency arises where one nonkey attribute determines another - a department code determining a department name in an employee relation. The dependent attribute then belongs with the determinant in a relation of its own, and removing such dependencies is what takes a relation from second to third normal form.

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22.

A relation has which of the following properties?

A.Each row is not unique.
B.Attributes can have the same name within a given table.
C.Each relation has a unique name.
D.The order of the columns is significant.

Answer: C

Correct answer: Each relation has a unique name.

Unique naming is a genuine property. The other three are each stated backwards: every row must be unique, no two attributes within a relation may share a name, and the order of columns is not significant - a relation is a set of tuples over named attributes rather than an ordered table.

Discuss in Forum

23.

A composite attribute does not get mapped into a relation.

A.True
B.False

Answer: A

Correct answer: True

The composite attribute itself becomes no column. What is mapped are its simple components - street, town and postcode rather than a single address - each becoming an attribute of the relation. The composite exists in the conceptual model as a grouping and disappears in the mapping.

Discuss in Forum

24.

An enterprise key is a primary key whose value is unique for a given relation.

A.True
B.False

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.

Discuss in Forum

25.

A primary key may be null.

A.True
B.False

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.

Discuss in Forum

26.

The entity integrity rule states that:

A.no primary key attribute may be null.
B.no primary key can be composite.
C.no primary key may be unique.
D.no primary key may be equal to a value in a foreign key.

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.

Discuss in Forum

27.

A candidate key must uniquely identify each row.

A.True
B.False

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.

Discuss in Forum

28.

When mapping a multivalued attribute into a relation which of the following is true?

A.One relation is created.
B.Two relations are created.
C.Three relations are created.
D.Four relations are created.

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.

Discuss in Forum

29.

When mapping a binary many-to-many relationship into a relation which of the following is true?

A.One relation is created.
B.Two relations are created.
C.Three relations are created.
D.Four relations are created.

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.

Discuss in Forum

30.

When mapping a ternary relationship with an associative entity into a relation which of the following is true?

A.One relation is created.
B.Two relations are created.
C.Three relations are created.
D.Four relations are created.

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.

Discuss in Forum

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