GATE CSE 2002 | Question: 1.20

Question

With regards to the expressive power of the formal relational query languages, which of the following statements is true?

• A. Relational algebra is more powerful than relational calculus
• B. Relational algebra has the same power as relational calculus
• C. Relational algebra has the same power as safe relational calculus
• D. None of the above

Comments

What if we compare sql and relational algebra then ??

---

@jlimbasiya

SQL is having more power than RA or TRC. Because aggregate , grouping , ordering all these can be done by SQL but can not be done by RA or TRC queries.

RA→ Relational algebra.

TRC→ Tuple relational calculus.

---

Good examples @raja11sep :)

Answer 1

Answer: C

Relational algebra has the same power as safe relational calculus as:

• A query can be formulated in safe Relational Calculus if and only if it can be formulated in Relational Algebra.

Comments

Reference ?

---

Difference between safe relational calculus and relational calculus

---

you may find an answer in korth book

Answer 2

Option "B" is also correct but option "C" is more appropriate because Relational calculus have some unsafe operations and because of those unsafe operations sometimes it may fall in ∞(infinite) loop.

so relational algebra has the same power as safe relational calculus

Answer 3

Reference:
http://www.cs.sfu.ca/CourseCentral/354/zaiane/material/notes/Chapter3/node15.html

Answer 4

A query can be written in safe Relational Calculus if and only if it can be written in Relational Algebra means it can be written in Relational Calculus as well as Relational Algebra, but Relational calculus also have some unsafe operations and because of those unsafe operations sometimes create ∞(infinite) loop.

So we can say that all safe relational calculus query can be translate into relational algebra

So Option (C) is the correct answer

Comments

Sir can you tell me, if we allow unsafe expression then which one is more powerful, is this comparison valid? Please help me