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Previous Year Question (PYQs)
Which of the following has same expressive power with regard to relational query language?
$(a)$ Relational algebra and domain relational calculus
$(b)$ Relational algebra and tuple relational calculus
$(c)$ Relational algebra and domain relational calculus restricted to safe expression
$(d)$ Relational algebra and tuple relational calculus restricted to safe expression
Solution
Relational algebra has the same expressive power as relational calculus only when relational calculus expressions are restricted to safe expressions.
Safe expressions ensure that the query result is finite and depends only on the database.
So,
Relational algebra is equivalent to domain relational calculus restricted to safe expressions.
Also,
Relational algebra is equivalent to tuple relational calculus restricted to safe expressions.
Therefore, $(c)$ and $(d)$ are correct.
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