A ____ key specifies a uniqueness constraint that no two distinct tuples in any state r of relation R can have the same value for super key.
Answer: D
Correct answer: Super
A super key is any set of attributes whose values are unique across the relation. A candidate key is a super key with no redundant attribute in it, so every candidate key is a super key but not the reverse.
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The fact that all employees of a particular organization should not have salaries more than $10000 is a _____ constraint.
Answer: C
Correct answer: Domain
A limit on the values a single attribute may take is a domain constraint - here, that salary must not exceed $10000. A tuple constraint would relate two attributes of the same row, and a schema constraint the structure itself.
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The data in the database at a particular moment of time is called the _______
Answer: A
Correct answer: Snapshot
The state of the database at one instant is a snapshot, or an instance. The schema, by contrast, is the description that holds across all such instants.
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The fact that information in the data warehouse changes far less often and may be regarded as non-real-time with periodic updating; This implies that data warehouses are _______
Answer: B
Correct answer: Non - Volatile
A data warehouse is loaded periodically and then read; existing data is not overwritten by day-to-day transactions. That is what non-volatile means here, and it is what allows historical comparisons that an operational database cannot support.
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Employees in a company are each assigned their own office, i.e. for each employee there exists a unique office and for each office there exists a unique employee; This statement is an example of ____ type of relation.
Answer: A
Correct answer: 1:1
One employee to one office and one office to one employee is a one-to-one relationship. The test is to read the mapping in both directions: if either direction allows several, the relationship is one-to-many or many-to-many.
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Which of the following relational algebra operations do not require the participating tables to be union-compatible?
Answer: D
Correct answer: Join
Union, intersection and difference are set operations, so both operands must have the same number of attributes drawn from the same domains - that is what union-compatible means. A join combines attributes from two different relations and so has no such requirement.
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Which of the following is not a property of transactions?
Answer: B
Correct answer: Concurrency
The properties of a transaction are the ACID four: Atomicity, Consistency, Isolation and Durability. Concurrency is something the DBMS manages between transactions, not a property of one.
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Relational Algebra does not have
Answer: C
Correct answer: Aggregation operators.
Pure relational algebra has selection, projection, union, difference, product, join and division, but no way to compute a sum or an average - aggregation is an extension added for practical query languages. This is why SQL is more than a direct implementation of the algebra.
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Checkpoints are a part of
Answer: A
Correct answer: Recovery measures.
A checkpoint records that everything up to a point has been written to disk, so recovery after a crash need only examine the log from that point onwards instead of from the beginning.
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Tree structures are used to store data in
Answer: C
Correct answer: Hierarchical model.
The hierarchical model arranges records in a tree, each child having exactly one parent. The network model uses a graph, which allows several parents, and the relational model uses tables with no structural links at all.
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