Data independence means
Answer: D
Correct answer: both B and C
Data independence has two halves: physical independence means programs do not depend on how the data is stored, and logical independence means they survive changes to the logical structure. Both are meant by the term, which is why the two options are taken together.
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Which of the following is not a consequence of non-normalized database?
Answer: D
Correct answer: Lost update problem
Update anomalies, insertion anomalies and redundancy are all consequences of a badly normalised design and would appear with a single user. The lost update problem arises from two transactions running at the same time, which is a concurrency issue.
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According to the levels of abstraction, the schema at the intermediate level is called
Answer: D
Correct answer: Logical schema.
The three levels of abstraction are physical at the bottom, logical in the middle and external, or view, at the top. The logical schema describes the whole database as tables and relationships without saying how it is stored.
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What are the desirable properties of a decomposition
Answer: B
Correct answer: Dependency preservation.
A decomposition should be lossless - the original relation must be recoverable by joining the pieces - and dependency preserving, so that every functional dependency can still be enforced on a single one of the resulting relations. Redundancy is what decomposition removes, not a property it should have.
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______is a preferred method for enforcing data integrity
Answer: A
Correct answer: Constraints
Constraints are declared as part of the table definition, so the DBMS itself enforces them on every operation and no path can bypass them. A trigger can enforce a rule too, but it is code that has to be written correctly and runs after the fact.
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DFD stands for
Answer: C
Correct answer: Data Flow Diagram
A data flow diagram shows the processes of a system, the data stores they read and write, and the flows between them - a functional picture, as against the entity-relationship diagram's picture of the data itself.
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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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