Relational Algebra,Tuple Calculus And SQL (Basic Level)- 1


10 Questions MCQ Test Question Bank for GATE Computer Science Engineering | Relational Algebra,Tuple Calculus And SQL (Basic Level)- 1


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QUESTION: 1

The relational algebra expression equivalent to the following expression:
 is:

Solution:

The expression

can be represented in relational algebra as

It says that select those rows from relation r where A = 10 union where B = 20.
Hence option (b) is correct.

QUESTION: 2

A relation (from the relational database model) consists of a set of tuples, which implies that

Solution:
QUESTION: 3

Which of the following statements is correct with respect to entity integrity?

Solution:

According to entity integrity primary key of relation should not contain null values.

QUESTION: 4

Which of the following operations is not part of the five basic set operations in relational algebra?

Solution:

Five primitive operators of Codd’s algebra are the selection, the projection the Cartesian product, the set union and the set difference.

QUESTION: 5

Which of the following relational algebraic operation is not a commutative operation?

Solution:
QUESTION: 6

Consider a banking database with the following relation with three attributes loans (br_name, loan_no, amount).
Find the appropriate query for the given statements below:
“Find the loan number for each loan of an amount greater than 20000”

Solution:

Loans (br_name, loan_no, amount)
For the query “find the loan number for each loan of an amount greater than 20000”.
We have to check for each loan to find a loan number which is greater than 20,000.

Hence (b) is the correct option.

QUESTION: 7

Let R(a, b, c) and S(d, e, f) be two relations in which d is the foreign key of S that refers to the primary key of R. Consider the following four operations R and S
1. Insert into R
2. Insert into S
3. Delete from R
4. Delete from S
Which of the following is true about the referential integrity constraint above?

Solution:

Referential integrity constraint: In relational model, two relation are related to each other over the basis of attributes, Every value of referencing attribute must be null or be available in the referenced attribute.

Here d is the foreign key of S that refers to the primary key of R.
1. Insert into R will not cause any violation.
2. Insert into S may cause violation because for each entry in ‘S ’ it must be. in ‘R ’ .
3. Delete from R may cause violation because for the deleted entry in R there may be referenced entry in the reIation S.
4. Delete from S will not cause any violation. 
Hence (d) is the correct option.

QUESTION: 8

Let r and s be two relations over the relation schemes R and S respectively, and let A be an attribute in R. Then the relational algebra expression   is always equal to 

Solution:

The relational algebra expression:

Hence (c) is the required option.

QUESTION: 9

Join Selectivity of a relation R in a natural join with a relation S is the _____

Solution:
QUESTION: 10

Which of the following is wrong?

Solution:

  is not equivalent because θ join is not cumulative hence the entries in   may not be same. Hence option (d) is correct.