Question

Choose all of the correct answers for each of the following:

[1] The number of superkeys of a relation R(A, B, C, D) with keys {A} and {B, C} is
1-2..
2- 10..
3- 12..
4- All of the above..
5- None of the above..
[2] The relations of two subentities of the same entity set have
1- The same keys..
2- The same attributes..
3- The same functional dependencies..
4- The same natural joins with their super entity set..
5- All of the above..
6- None of the above..
[3] A relation R
1- Is in BCNF if R is in 3NF..
2- Is in 3NF if R is in BCNF..
3- May be in BCNF but not in 3NF..
4- May be in 3NF but not in BCNF..
5- May be neither in BCNF nor in 3NF..
6- All of the above..
7- None of the above..
[4] The natural join of relations R and S may be expressed as
1- (oc(R x S))..
2- 0ca_ (R x S))..
3- Neither "1" nor "2"..
4- Either "1" or "2"..
[5] A tuple constraint that references an attribute R.A to attribute S.B is checked
1- When changes in R are made..
2- When changes in R.A are made..
3- When changes in S are made..
4- When changes in S.B are made..
5- Any of the above..
6- None of the above..

Answer 1

[1] = 10

[2] = The same natural joins with their super entity set

[3] = Is in 3NF if R is in BCNF

[4] = Neither "1" or "2"

[5] = When changes in R.A are made

Step-by-step explanation:

[1] The number of super keys of a relation R(A, B, C, D) with keys {A} and {B, C} is 10. The super keys of the given relations are {A}; {A, B}; {A,C}; {A,D}; {A,B,C}; {A,B,D}; {A,C,D}; {A,B,C,D}; {B,C}; {B,C,D}.

[2] key is used to uniquely identify entity from entity set. So key for every entity is unique. Subentities can have different attributes. Also functional dependencies can be different but natural join with their super subsets will be same.

[3] For a relation R to be in BCNF it must be in 3NF but If a relation R is in 3NF it is not necessary that it will be in BCNF.

[4] The natural join of relations R and S is expressed by R ✕ S.

[5] If tuple constraint references an attribute R.A to S.B then every time a change is done in R.A the tuple constraint is checked.