A tuple is a(n):
Answer: C
Correct answer: row of a table.
A tuple is a row, the record of the older vocabulary. Since no two rows in a relation may be identical and the order of rows carries no meaning, a relation is formally a set of tuples - which is why a query result has no inherent ordering unless one is requested explicitly.
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A column that forms part of a foreign key can never contain a NULL value.
Answer: B
Correct answer: False
Referential integrity requires only that a non-null foreign key match an existing primary key value. A NULL is permitted and simply means the relationship is unknown or does not apply, unless the column is separately declared NOT NULL.
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A candidate key is a determinant that determines all the other columns is a relation.
Answer: A
Correct answer: True
A candidate key is a minimal set of attributes that uniquely identifies a row, and identifying the row fixes the value of every attribute in it - so the key functionally determines every other column. This is why candidate keys are determinants; the point of Boyce-Codd Normal Form is to insist that every determinant be one of them.
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One solution to the multivalued dependency constraint problem is to:
Answer: A
Correct answer: split the relation into two relations, each with a single theme.
A multivalued dependency problem arises when one relation holds two independent multivalued facts about the same key, forcing every combination of the two to be stored. Separating them into two relations, each carrying a single theme, removes the spurious combinations entirely - and this is exactly what putting the relation into fourth normal form means.
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In a relation, the rows are sometimes called "fields."
Answer: B
Correct answer: False
Rows are tuples, or records; it is the columns that are called fields, along with attributes. Getting the correspondence the wrong way round is easy, so it is worth fixing the two triples in mind: relation, table, file for the whole structure; tuple, row, record across; and attribute, column, field down.
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A relation is in Boyce-Codd Normal Form (BCNF) if every determinant is a composite key.
Answer: B
Correct answer: False
Boyce-Codd Normal Form requires that every determinant be a candidate key, and says nothing about whether that key is composite. A single-attribute key is perfectly acceptable. The condition exists because a determinant that is not a candidate key allows the same fact to be stored in many rows, which is the source of update anomalies.
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A relation is in this form if it is in BCNF and has no multivalued dependencies:
Answer: C
Correct answer: fourth normal form.
Fourth normal form adds the elimination of multivalued dependencies to the requirements of Boyce-Codd Normal Form. A multivalued dependency arises when one attribute determines a set of values rather than a single value, and two such independent sets in one relation force every combination to be stored - which is what splitting into separate relations removes.
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In a relation, the order of the rows matters.
Answer: B
Correct answer: False
Row order carries no meaning in a relation: a relation is a set of tuples, and a set has no inherent sequence. A query that needs its output in a particular order must request it explicitly, and any apparent ordering in a result set without such a request is an accident of storage and must not be relied on.
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Given the functional dependency R → (S,T) , then it is also true that R → S.
Answer: A
Correct answer: True
If R determines the pair (S, T), then given a value of R the value of S is fixed - the pair could not be determined otherwise. This is the decomposition rule of Armstrong’s axioms, and it is what allows a dependency on a set of attributes to be broken down into a dependency on each of them separately. The converse does not follow.
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A relation has cells that hold multi-value entries.
Answer: B
Correct answer: False
One of the defining conditions of a relation is that each cell contains a single value. A cell holding several values - a list of phone numbers, say - means the table is not in first normal form and is not a relation at all. The remedy is to move the repeating values into a separate relation.
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