Which of the following has same expressive power with regard to relational…
2019
Which of the following has same expressive power with regard to relational query language?
(a) Rational algebra and domain relational calculus
(b) Relational algebra and tuples relational calculus
(c) Relational algebra and domain relational calculus restricted to safe expression
(d) Relational algebra and tuples relational calculus restricted to safe expression
- A.
(a) and (b) Only
- B.
(c) and (d) Only
- C.
(a) and (c) Only
- D.
(b) and (d) Only
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Correct answer: B
Answer: Relational algebra has the same expressive power as both domain relational calculus restricted to safe (domain-independent) expressions and tuple relational calculus restricted to safe expressions.
Relational algebra: a procedural query language that returns finite relations determined solely by the database.
Tuple relational calculus and domain relational calculus: declarative query languages. Their unrestricted forms can express unsafe or domain-dependent queries that may yield infinite or non–database-dependent results.
Safe (domain-independent) expressions: restrict calculus so results depend only on the actual database and are finite; with this restriction the calculi match the expressive power of relational algebra.
Therefore, the two correct statements are the ones that pair relational algebra with domain relational calculus restricted to safe expressions and relational algebra with tuple relational calculus restricted to safe expressions.
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