Practice important DBMS Keys and ER Model MCQ questions with answers and explanations. These questions cover concepts like primary key, foreign key, candidate key, super key, ER modeling, attributes, and cardinality. Useful for IBPS IT Officer, GATE, and other competitive exams.
Difficulty: Easy to Hard
Total Questions: 15
Important DBMS Keys and ER Model MCQ Questions
Q1. Which of the following best describes a Super Key?
A. A minimal set of attributes that uniquely identifies a tuple
B. Any set of attributes that uniquely identifies a tuple
C. A key with only one attribute
D. A foreign key in another table
View Answer & Explanation
Answer: B
Explanation: A super key uniquely identifies tuples but may contain redundant attributes. Candidate keys are minimal super keys.
Q2. A Candidate Key is:
A. A super key with minimal attributes
B. A foreign key
C. Always composite
D. Always numeric
View Answer & Explanation
Answer: A
Explanation: A candidate key is a minimal super key with no redundant attributes.
Q3. Which key is selected from candidate keys to uniquely identify tuples?
A. Foreign Key
B. Primary Key
C. Composite Key
D. Alternate Key
View Answer & Explanation
Answer: B
Explanation: One candidate key is chosen as the primary key.
Q4. A Foreign Key is used to:
A. Uniquely identify records
B. Link two tables together
C. Store duplicate values
D. Remove redundancy
View Answer & Explanation
Answer: B
Explanation: A foreign key references a primary key in another table to maintain referential integrity.
Q5. Which of the following can be a Primary Key?
A. NULL values
B. Duplicate values
C. Unique and NOT NULL values
D. Only numeric values
View Answer & Explanation
Answer: C
Explanation: Primary keys must be unique and cannot contain NULL values.
Q6. If a table has multiple candidate keys, then the unused candidate keys are called:
A. Super Keys
B. Alternate Keys
C. Composite Keys
D. Foreign Keys
View Answer & Explanation
Answer: B
Explanation: Candidate keys not selected as primary keys are alternate keys.
Q7. Which of the following is TRUE?
A. Every super key is a candidate key
B. Every candidate key is a super key
C. Every foreign key is a primary key
D. Every primary key is composite
View Answer & Explanation
Answer: B
Explanation: All candidate keys are super keys, but not all super keys are minimal.
Q8. In an ER model, an entity is best described as:
A. A relationship between tables
B. A real-world object distinguishable from others
C. A column in a table
D. A constraint
View Answer & Explanation
Answer: B
Explanation: An entity represents a real-world object like Student, Employee, etc.
Q9. Which of the following is a composite attribute?
A. Age
B. Address
C. Salary
D. ID
View Answer & Explanation
Answer: B
Explanation: Address can be divided into street, city, state, etc.
Q10. A weak entity is one that:
A. Has no attributes
B. Cannot exist without a strong entity
C. Has only one attribute
D. Is not stored in database
View Answer & Explanation
Answer: B
Explanation: Weak entities depend on strong entities for identification.
Q11. Cardinality in ER model refers to:
A. Number of attributes
B. Number of relationships between entities
C. Type of key used
D. Data type of attribute
View Answer & Explanation
Answer: B
Explanation: Cardinality defines how many instances of one entity relate to another.
Q12. Which of the following violates Entity Integrity?
A. Primary key is NULL
B. Foreign key is NULL
C. Attribute is composite
D. Attribute is derived
View Answer & Explanation
Answer: A
Explanation: Primary keys must be unique and NOT NULL.
Q13. Which attribute type can have multiple values for a single entity?
A. Simple attribute
B. Derived attribute
C. Multi-valued attribute
D. Key attribute
View Answer & Explanation
Answer: C
Explanation: Multi-valued attributes can store multiple values like phone numbers.
Q14. If a relation has two candidate keys, what is the minimum number of super keys?
A. 2
B. 3
C. 4
D. More than 2
View Answer & Explanation
Answer: D
Explanation: Every candidate key is a super key, and adding attributes creates more super keys, so count is more than 2.
Q15. Which of the following correctly represents a one-to-many relationship?
A. One student has many IDs
B. One department has many employees
C. One employee has many departments
D. One key has many primary keys
View Answer & Explanation
Answer: B
Explanation: One department can have multiple employees, but each employee belongs to one department.
Conclusion
These DBMS Keys and ER Model MCQs help strengthen understanding of keys, relationships, and database design concepts. Practicing such mixed-difficulty questions improves conceptual clarity and exam performance.
For detailed theory, refer to DBMS Keys and ER concepts .