Keys
Question 1 |
Consider the following statements S1 and S2 about the relational data model:
S1: A relation scheme can have at most one foreign key.
S2: A foreign key in a relation schema R cannot be used to refer to tuples of R.
Which one of the following choices is correct?
S1: A relation scheme can have at most one foreign key.
S2: A foreign key in a relation schema R cannot be used to refer to tuples of R.
Which one of the following choices is correct?
S1 is false and S2 is true. | |
Both S1 and S2 are false. | |
Both S1 and S2 are true.
| |
S1 is true and S2 is false. |
Question 1 Explanation:
- A database table may have more than one foreign key, and each foreign key can have a different parent table. Hence, the statement I is incorrect.
- A foreign key is a set of attributes in a table that refers to the primary key of another table or to the primary key of the same table (self-referential table). Hence, the statement II is also incorrect.
Question 2 |
A primary key for an entity is
A candidate | |
Any attribute
| |
A unique attribute
| |
A super key |
Question 2 Explanation:
A primary key for an entity is a candidate key.
Question 3 |
A super key for an entity consists of
one attribute only
| |
At least two attributes
| |
at most two attributes
| |
one or more attributes
|
Question 3 Explanation:
Super key is a set of attributes of a relation schema upon which all attributes of the schema are functionally dependent. It can consist of one or more attributes.
For example : Consider a relation R(A,B,C,D) where “A” is the candidate key of “R” then Superkeys of “R” will be {A,AB, AC, AD, ABC, ABD, ACD, ABCD}.
For example : Consider a relation R(A,B,C,D) where “A” is the candidate key of “R” then Superkeys of “R” will be {A,AB, AC, AD, ABC, ABD, ACD, ABCD}.
Question 4 |
A ____ of a relation is a set of one or more attributes whose values are guaranteed to identify tuples in the relation uniquely.
Foreign key | |
Super key
| |
Scheme | |
key
|
Question 4 Explanation:
Superkey is the superset of the candidate keys or keys having one or more attributes whose values are guaranteed to identify tuples in the relation uniquely.