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Consider the following relational schema.

• Students(rollno: integer, sname: string)
• Courses(courseno: integer, cname: string)
• Registration(rollno: integer, courseno: integer, percent: real)

Which of the following queries are equivalent to this query in English?

“Find the distinct names of all students who score more than $90\%$ in the course numbered 107”

1. SELECT DISTINCT S.sname FROM Students as S, Registration
as R WHERE
R.rollno=S.rollno AND R.courseno=107 AND R.percent >90

2. $∏_{sname}(σ_{courseno=107 ∧ percent > 90} (Registration ⋈ Students))$
3. $\{T \mid ∃S \in Students, ∃R \in Registration ( S.rollno=R.rollno$
$∧ R.courseno=107 ∧ R.percent>90 ∧T.sname=S.sname) \}$
4. $\left\{ \langle S_N\rangle \mid ∃S_R∃R_P (\langle S_R,S_N\rangle ∈Students ∧ \\ \langle S_R,107,R_P\rangle ∈Registration ∧ R_P>90) \right\}$
1. I, II, III and IV
2. I, II and III only
3. I, II and IV only
4. II, III and IV only

Why not T.sname in 3rd option? It is causing hell a lot of confusion. In every other TRC question the attribute to be printed is given in LHS of |.

Please someone explain the IV query.
TRC & DRC removes duplicate result same as the DISTINCT keyword in SQL.

Four queries given in SQL, RA, TRC and DRC in four statements respectively retrieve the required information.

For people having problem that why option 3 is corect as it is printing whole tuple... actually it is not printing whole tuple  ,T.sname=S.sname will print the student name from student table  ....(T is a free variable )
Thanks a lot for this !
T is a free variable but to print only student names we should have T.Sname on the left hand side of the | instead of T.

If T itself returns student names then T.sname dose not make any sense. It is like writing sname.sname.
Option A:

This is a SQL query expression. It first perform a cross product of Students
and Registration, then WHERE clause only keeps those rows in the cross product
set where the student is registered for course no 107, and percentage is > 90.
Then select distinct statement gives the distinct names of those students as the
result set.


Option B:

This is a relational algebra expression. It first perform a NATURAL JOIN
of Students and Registration (NATURAL JOIN implicitly joins on the basis
of common attribute, which here is rollno ), then the select operation( sigma)
keeps only those rows where the student is registered for courseno 107,
and percentage is > 90. And then the projection operation (pi) projects only
distinct student names from the set.

Note: Projection operation (pi) always gives the distinct result.

Option C:

This is a Tuple Relational Calculus (TRC) language expression,
It is not a procedural language (i.e. it only tells “what to do”,
not “how to do”). It just represents a declarative mathematical
expression.

Here T is a Tuple variable.

From left to right, it can be read like this, “It is a set of
tuples T, where, there exists a tuple S in Relation Students, and
there exist a tuple R in relation Registration, such that
S.rollno = R.rollno AND R.couseno = 107 AND R.percent > 90 AND
T.sname = S.sname”. And the schema of this result is (sname), i.e. each
tuple T will contain only student name, because only T.sname has been defined
in the expression.

As TRC is a mathematical expression, hence it is expected to give only distinct result set.

Option D:

This is a Domain Relational Calculus (DRC) language expression.
This is also not procedural. Here SN is a Domain Variable. It can be read
from left to right like this “The set of domain variable SN, where,
there exist a domain variable SR , and a domain variable Rp, such that,
SN and SR domain variables is in relation Students and SR,107,RP is a domain
variables set in relation Registration, AND RP > 90 “

Above, SN represents sname domain attribute in Students relation, SR
represents rollno domain attribute in Students relation, and RP represents
percentage domain attribute in Registration relation.
The schema for the result set is (SN), i.e. only student name.

As DRC is a mathematical expression, hence it is expected to
give only distinct result set.

So clearly all options are correct.

by

### 1 comment

Geeks for geeks 😂😂