RDBMS test on “Decomposition Using Functional Dependencies”.
1. An instance of a relation that satisfies all real world constraints is known as?
a) Proper relation
b) Ideal relation
c) Perfect relation
d) Legal relation
Answer: d
Clarification: A relation that satisfies all the real world constraints is called as a legal relation. An instance of a legal relation is called as a legal instance.
2. If K → R then K is said to be the _______ of R
a) Candidate key
b) Foreign key
c) Super key
d) Domain
Answer: c
Clarification: If K → R then k is said to be the superkey of R i.e. K uniquely identifies every tuple in the relation R.
3. X → Y holds on a schema k(K) if?
a) At least one legal instance satisfies the functional dependency
b) No legal instance satisfies the functional dependency
c) Each and every legal instance satisfies the functional dependency
d) None of the mentioned
Answer: c
Clarification: X → Y holds on a schema k(K) if each and every legal instance satisfies the functional dependency. Even if one instance does not satisfy the functional dependency X→ Y does not hold on a schema.
4. X→ Y is trivial if?
a) X ⊂ Y
b) Y ⊂ X
c) X ⊇ Y
d) None of the mentioned
Answer: a
Clarification: X→ Y is said to be trivial if X is a subset of Y. Thus X ⊂ Y implies X→Y is trivial.
5. Which of the following is not a condition for X→ Y in Boyce codd normal form?
a) X → Y is trivial
b) X is the superkey for the relational schema R
c) Y is the superkey for the relational schema R
d) All of the mentioned
Answer: c
Clarification: Y does not need to be a superkey of the relation for the given functional dependency to satisfy BCNF. X→ Y must be trivial and X must be the superkey of the relation R.
6. Which of the following is used to express database consistency?
a) Primary keys
b) Functional dependencies
c) Check clause
d) All of the mentioned
Answer: d
Clarification: Primary keys, Functional dependencies, Check clause are all used to express database consistency.
7. Which of the following is not a condition for the third normal form in the case of XY?
a) X→ Y is trivial
b) X is the superkey for R
c) Each attribute in Y-X is a candidate key for R
d) Each attribute in X-Y is a candidate key for R
Answer: d
Clarification: Each attribute in X-Y is a candidate key for R is not a necessary condition for the third normal form in the case of X → Y. X→Y is trivial and X is the superkey of R are necessary conditions for the third normal form.
8. F+ is called as the ________ of F
a) Closure
b) Sum
c) Cartesian product
d) None of the mentioned
Answer: a
Clarification: F+ is called as the closure of F. It contains all the possible combinations of a functional dependency set in it.
9. State true or false: A functional dependency must first satisfy the second normal form to satisfy the third normal form.
a) True
b) False
Answer: a
Clarification: A functional dependency must first satisfy the second normal form to satisfy the third normal form. The second normal form must in turn satisfy first normal form.
10. State true or false: The fourth normal form does not exist and it is instead called as the BCNF.
a) True
b) False
Answer: b
Clarification: The fourth normal form exists and is different from the Boyce Codd normal form which is called as BCNF.