A relation has which of the following properties?
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.
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A composite attribute does not get mapped into a relation.
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.
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An enterprise key is a primary key whose value is unique for a given relation.
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.
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A primary key may be null.
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.
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The entity integrity rule states that:
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.
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A candidate key must uniquely identify each row.
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.
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When mapping a multivalued attribute into a relation which of the following is true?
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.
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When mapping a binary many-to-many relationship into a relation which of the following is true?
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.
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When mapping a ternary relationship with an associative entity into a relation which of the following is true?
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.
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Well-structured relations encourage anomalies of data.
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.
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