50 DBMS MCQs with Answers

Question 11

A relation r(A,B) in a relational database has 1200 tuples. The attribute A has integer values ranging from 6 to 20, and the attribute B has integer values ranging from 1 to 20. Assume that the attributes A and B are independently distributed. The estimated number of tuples in the output of σ_{(A>10)∨(B=18)}(r) is ____________.

• 820
• 1200
• 960
• 1000

Question 12

Relations produced from E - R Model will always be in ________.

• 1 NF
• 2 NF
• 3 NF
• 4 NF

Question 13

Consider the relation R (ABCDE): FD = { A → B, B → C, C → D, D → E} Find out the highest normal form.

• 1 NF
• 2 NF
• 3 NF
• BCNF

Question 14

Let R = ABCDE is a relational scheme with functional dependency set F = {A → B, B → C, AC → D}. The attribute closures of A and E are

• ABCD, φ
• ABCD, E
• Φ, φ
• ABC, E

Question 15

BCNF is not used for cases where a relation has

• Two (or more) candidate keys
• Two candidate keys and composite
• The candidate key overlap
• Two mutually exclusive foreign keys

Question 16

The Relation Vendor Order (V_no, V_ord_no, V_name, Qty_sup, unit_price) is in 2NF because:

• Non_key attribute V_name is dependent on V_no which is part of composite key
• Non_key attribute V_name is dependent on Qty_sup
• Key attribute Qty_sup is dependent on primary_key unit price
• Key attribute V_ord_no is dependent on primary_key unit price

Question 17

Which of the following concurrency control protocols ensure both conflict serializability and freedom from deadlock? I. 2-phase locking II. Time-stamp ordering

• I only

• II only

• Both I and II

• Neither I nor II

Question 18

Consider the following transactions with data items P and Q initialized to zero:

T1: read (P) ;
    read (Q) ;
    if P = 0 then Q : = Q + 1 ;
    write (Q) ;
T2: read (Q) ;
    read (P) ;
    if Q = 0 then P : = P + 1 ;
    write (P) ;

Any non-serial interleaving of T1 and T2 for concurrent execution leads to

• A serializable schedule

• A schedule that is not conflict serializable

• A conflict serializable schedule

• A schedule for which a precedence graph cannot be drawn

Question 19

Assume the following information: Original timestamp value = 46 Receive timestamp value = 59 Transmit timestamp value = 60 Timestamp at arrival of packet = 69 Which of the following statements is correct?

• Receive clock should go back by 3 milliseconds
• Transmit and Receive clocks are synchronized
• Transmit clock should go back by 3 milliseconds
• Receive clock should go ahead by 1 milliseconds

Question 20

Suppose a database system crashes again while recovering from a previous crash. Assume checkpointing is not done by the database either during the transactions or during recovery. Which of the following statements is/are correct?

• The same undo and redo list will be used while recovering again
• The system cannot recover any further
• All the transactions that are already undone and redone will not be recovered again
• The database will become inconsistent