Schedule S is R1(A), W1(A), R2(A), W2(A), R1(B), W1(B), R2(B), W2(B). Which serial schedule is S conflict-equivalent to?
Correct answer D. T1 then T2
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DBMS and SQL MCQs
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An unnormalised table records (student, course, course_fee). When the last student enrolled in a course withdraws and the row is deleted, the course fee is lost too. Which anomaly is this?
Correct answer A. Deletion anomaly
A Student table stores several phone numbers in one column as a comma-separated string such as ‘0300-1111111, 0321-2222222’. Which normal form does this violate?
Correct answer B. First normal form
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R(A, B, C) has functional dependencies AB -> C and C -> B, so its candidate keys are AB and AC. Which statement is true?
Correct answer A. R is in 3NF but not in BCNF
Emp(emp_id, dept_id, dept_name) has key emp_id, with emp_id -> dept_id and dept_id -> dept_name. What is the highest normal form this relation satisfies?
Correct answer B. 2NF
Enrol(student_id, course_id, student_name, grade) has key (student_id, course_id), with student_id -> student_name and (student_id, course_id) -> grade. Which normal form does it fail first?
Correct answer C. 2NF
Relation R(A, B, C, D, E) has functional dependencies A -> B, B -> C and CD -> E. Which of these is a candidate key of R?
Correct answer C. AD
Relation R(A, B, C, D, E) has functional dependencies A -> B, B -> C and CD -> E. What is the attribute closure {A}+?
Correct answer B. {A, B, C}
Which of these relational algebra operations is not one of the basic operations, because it can be expressed using a Cartesian product followed by a selection and projection?
Correct answer A. Natural join
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Relation R has 4 tuples and 3 attributes, and S has 5 tuples and 2 attributes, with no attribute names in common. How large is the Cartesian product R x S?
Correct answer D. 20 tuples, 5 attributes
Supply(s, p) contains (s1,p1), (s1,p2), (s2,p1), (s3,p1), (s3,p2), (s3,p3), and Parts(p) contains p1 and p2. What is Supply divided by Parts?
Correct answer A. {s1, s3}
Relation R(A, B) contains tuples (1,x), (1,y) and (2,x). How many tuples are in the relational algebra result of projection on A, written pi_A(R)?
Correct answer B. 2
For the relational algebra expression R union S to be valid, what must be true of R and S?
Correct answer A. They must have the same number of attributes with compatible domains
R(A, B, C) has the single functional dependency B -> C. Which decomposition of R is lossless-join?
Correct answer B. R1(A, B) and R2(B, C)
Which statement about decomposing a relation is always true?
Correct answer A. A lossless, dependency-preserving decomposition into 3NF always exists
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Which of the following SQL commands is a Data Definition Language (DDL) command?
Correct answer B. ALTER
You want to empty a large table of all its rows quickly while keeping the table structure for reuse, without row-by-row processing. Which command is designed for this?
Correct answer D. TRUNCATE TABLE t;
User ali was earlier given SELECT on table Emp. Which statement correctly withdraws that privilege?
Correct answer C. REVOKE SELECT ON Emp FROM ali;
What does adding WITH GRANT OPTION to GRANT SELECT ON Emp TO sara allow sara to do?
Correct answer B. Grant SELECT on Emp to other users
Inside a transaction you run two UPDATE statements and then realise they were wrong. Which command undoes all changes made since the transaction began?
Correct answer D. ROLLBACK