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1
GATE CSE 2004
MCQ (Single Correct Answer)
+2
-0.6
Consider the relation Student (name, sex, marks), where the primary key is shown underlined, pertaining to students in a class that has at least one boy and one girl. What does the following relational algebra expression produce? (Note: ρ is the rename operator). GATE CSE 2004 Database Management System - Relational Algebra Question 22 English
A
names of girl students with the highest marks
B
names of girl students with more marks than some boy student
C
names of girl students with marks not less than some boy student
D
names of girl students with more marks than all the boy students
2
GATE CSE 2001
MCQ (Single Correct Answer)
+2
-0.6
Which of the following relational calculus expressions is not safe?
A
$$\left\{t \mid \exists u \in R_1\left(t[A] = u[A]\right) \land \neg \exists s \in R_2 \left(t[A] = s[A]\right)\right\}$$
B
$$\left\{ t \mid \forall u \in R_1\left(u[A]="x"\Rightarrow \exists s \in R_2\left(t[A] = s[A] \land s[A] = u[A]\right)\right) \right\}$$
C
$$\left\{t \mid \neg (t \in R_1)\right\}$$
D
$$\left\{t \mid \exists u \in R_1\left(t[A]=u[A]\right) \land \exists s \in R_2 \left(t[A] = s[A]\right)\right\}$$
3
GATE CSE 1998
MCQ (Single Correct Answer)
+2
-0.6
Which of the following query transformations (i.e. replacing the l.h.s. expression by the r.h.s. expression) is incorrect? R1 and R2 are relations, C1, C2 are selection conditions and A1, A2 are attributes of R1?
A
$$\sigma_{C_1} \left(\sigma_{C_2}\left(R_1\right)\right) \to \sigma_{C_2} \left(\sigma_{C_1}\left(R_1\right)\right)$$
B
$$\sigma_{C_1} \left(\pi_{A_1}\left(R_1\right)\right) \to \pi_{A_1} \left(\sigma_{C_1}\left(R_1\right)\right)$$
C
$$\sigma_{C_1} \left(R_1 \cup R_2\right) \to \sigma_{C_1}\left(R_1\right) \cup \sigma_{C_1} \left(R_2\right)$$
D
$$\pi_{A_1} \left(\sigma_{C_1}\left(R_1\right)\right) \to \sigma_{C_1} \left(\pi_{A_1}\left(R_1\right)\right)$$
4
GATE CSE 1996
Subjective
+2
-0

A library relational database system uses the following schema

USERS (User #, User Name, Home Town)

BOOKS (Books # Book Title, Author Name)

ISSUED (Book #, User #, Date)

Explain in one English sentence, what each of the following relational algebra queries is designed to determine.

(a) $$\sigma_{ \text{User#}=6}\left(\pi_{\text{User#, Book Title}}\left(\left(\text{USERS} \bowtie \text{ISSUED}\right) \bowtie \text{BOOKS}\right)\right)$$

(b) $$\pi_{\text{Author Name}}\left(\text{BOOKS} \bowtie \sigma_{\text{Home Town=Delhi}} \left(\text{USERS} \bowtie \text{ISSUED}\right)\right)$$
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