DBMS: Solved 518 DBMS Questions and answers section with explanation for various online exam preparation, various interviews, Logical Reasoning Category online test. Category Questions section with detailed description, explanation will help you to master the topic.

DBMS Questions

1. Which of the following scenarios may lead to an irrecoverable error in a database system ?




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Correct Ans:A transaction reads a data item after it is written by an uncommitted transaction
Explanation:
A transaction reads a data item after it is written by an uncommitted transaction.


2. Consider the following relational schema pertaining to a student's database:

Students (rollno, name, address )
Enroll( rollno, courseno, coursename)

Where primary keys are shown in italics. The number of tuples in the student and Enroll tables are 120 and 8 respectively. What are the maximum and minimum number of tuples that can be present in (Student *Enroll) , where * denotes natural join ?




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Correct Ans:8,8
Explanation:
* denotes the natural join So, maximum and minimum numbers of tuples that can be present in (student * Enroll) would be represented by the minimum of these min (120,8) = 8


3. Which of the following is a key factor for preferring B+ tree to binary search trees for indexing database relations ?




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Correct Ans:data transfer from disk is in blocks
Explanation:
Indexing is better if data blocks is large, Now, B+ trees are preferred over the binary search trees as in B+ trees, transfer of data from disk to primary memory is in form of data blocks. These transferring , information on a single block is more efficient


4. Which of the following concurrency control protocols ensure both conflict serializability and freedom from deadlock ?
1. 2 phase locking
2. time stamp ordering




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Correct Ans:2 only
Explanation:
Time stamp ordering ensures both conflict serializability and freedom from deadlock as time-stamp based concurrency control is a non lock concurrency control method. The method is employed in relation databases to safely handle transaction. It uses timestamp for the same.


5. Consider a B+ tree in which the maximum number of keys in a node is 5. What is the minimum number of keys in any non root node ?




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Correct Ans:2
Explanation:
In b+ tree root node has minimum 2 block pointers and maximum p block pointer where, p= order where key = order-1 and in the non root node, the minimum number of keys = p/2 - 1 so in the question key = 5, order= 6 So minimum number of keys in non root node = 6/2 -1 = 2


6. Consider a relational table with a single record for each registered student with the following attributes
1. Registration_number: Unique registration number for each registered student
2. UID: Unique identity number at the national level for each citizen
3. Bank Account_number: Unique account number at the bank. A student can have multiple accounts or join accounts.. This attributes stores the primary account number.
4. Name: Name of the student
5. Hostel_room: room number of the hostel

Which of the following option is incorrect ?




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Correct Ans:Bank Account_number is candidate key
Explanation:


7. ________ responsible for authorizing access to the database, for co-ordinating and monitoring its use, acquiring software, and hardware resources, controlling its use and monitoring efficiency of operations.




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Correct Ans:Buffer Manager
Explanation:


8. Which command is used to remove a table or index from the database in SQL?




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Correct Ans:DROP TABLE
Explanation:


9. Which command is used to remove rows from a table?




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Correct Ans:Both (a) & (b)
Explanation:


10. Transaction X holds a shared lock R. if transaction Y requests for a shared lock on R :




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Correct Ans:Immediately Granted
Explanation:


11. Which of the following is a relational database




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Correct Ans:4th Dimensions
Explanation:


12. Which of the following is the problem of file management system? (a) Lack of data independence(b) Data redundancy(c) Update (d) all Periodically adding, changing, & Deleting file records is called file




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Correct Ans:Renewing
Explanation:


13. For a database relation R(a, b, c, d) where the domains of a, b, c, d include only the atomic values. The functional dependency a ? c, b ? d holds in the following relation




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Correct Ans:In 2NF not in 3NF
Explanation:


14. The employee information of an Organization is stored in the relation : Employee (name, sex, salary, deptname) Consider the following SQL query Select deptname from Employee Where sex = 'M' group by deptname having avg (salary) > {select avg (salary) from Employee} Output of the given query corresponds to




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Correct Ans:Average salary of male employees in a department is more than average salary of the organization
Explanation:


15. The “PROJECT” operator of a relational algebra creates a new table that has always




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Correct Ans:Same number of rows as the original table
Explanation:


16. If D1,D2, ....Dn are domains in a relational model, then the relation is a table, which is a subset of




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Correct Ans:D1?D2? ... ?Dn
Explanation:


17. Consider a schema R(A, B, C, D) and functional dependencies A ? B and C ? D. Then the decomposition R1(A, B) and R2(C, D) is




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Correct Ans:Dependency preserving but not lossless join
Explanation:


18. B+ tree are preferred to binary tree in Database because




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Correct Ans:Disk access is much slower than memory access
Explanation:


19. In multiuser database if two users wish to update the same record at the same time, they are prevented from doing so by




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Correct Ans:Record lock
Explanation:


20. If a relation with a Schema R is decomposed into two relations R1 and R2 such that (R1 ? R2) = R1 then which one of the following is to be satisfied for a lossless joint decomposition (? indicates functional dependency)




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Correct Ans:(R1 ? R2) ? R1 or R1 ? R2 ? R2
Explanation:





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