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## DBMS Multiple Choice Questions - with Answers

### 1. Entity Relationship model

What type of relationship does the phrase “Teacher works in the school” express?
a) One-to-one
b) One-to-many
c) Many-to-one
d) Many-to-many
 Answer: (a) If we consider Teacher as one entity and school as another entity with works as the relationship, then with the assumption that a teacher can work at only one school at a time, the relationship set works is a one-to-one relationship.

### 2. Database models

In a Hierarchical database model, records are organized as _____ structure.
a) Graph.
b) List .
d) Tree.
 Answer: (d) A hierarchical database model is a data model in which the data are organized into a tree-like structure. The data are stored as records which are connected to one another through links.

3. Normalization
For a relation R(A, B, C, D, E) with set of functional dependencies F = {AB → E, D → C}, which of the following is a candidate key?
a) ABCD
b) ABD
c) BCD
d) ACD

### A candidate key is a minimal super key. That is, it is a super key for which no proper subset can be a key.

We can identify the candidate keys by finding the closure (A+) of set of one or more attributes.
In options (c) and (d), the only attributes on the LHS of given FD are BD and AD respectively. But, neither {BD}+ nor {AD}+ forms a key, because {BD}+ and {AD}+ are not equals to R.
In options (a) and (b), all attributes of LHS of F are present. Hence, both {ABCD}+ and {ABD}+ equals to R. So, both are super key.
As per the definition of candidate key (a super key is a candidate key if its subset does not constitute a key), ABCD is not a super key because the subset ABD is a key too. Hence, ABD is the candidate key.

### 4. Levels of abstraction

Which of the following database abstraction levels is closest to the users?
a) Physical level
b) Conceptual level
c) Internal level
d) External level
 Answer: (d) External level (view level) describes the user views and it is the closes database abstraction level.

### 5. Normalization

Consider a relation R(A, B, C, D, E) that satisfies the following set of functional dependencies F = {ABC → D, D → E, E → B, AD → C}. What is the highest normal form this table is currently in?

### c) Third Normal Form

d) None of the above

### BCNF – Left side of any FD should be a candidate key.

D is a determiner in the FD D → E. But D+ ≠ R. Hence, D is not a candidate key and the table is not in BCNF.

### 3NF – No non-key dependency or no transitive dependencies permitted.

E is an attribute that depends on a non-key attribute D. This is a transitive dependency. Hence, table not in 3NF.

### 2NF – No partial key dependencies permitted.

ABC is the key for R because {ABC}+ = R. Also, there exists no partial key dependencies. That is, no portion of the composite key alone determines another attribute in the given set F. Hence, the given table is in 2NF.

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# solved quiz questions with answer in relational model, super key, candidate key

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