What is the difference between a primary key and a unique key? MCQ with Answer and Explanation

Question

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What is the difference between a primary key and a unique key?

A. Primary key cannot have NULL; unique key can have one NULL (in most DBMS)

B. Unique key is always a foreign key

C. There is no difference

D. Primary key is always integer; unique key can be any data type

Answer: Option A

Solution (By JKSSB Mock Tests)

A primary key is a unique identifier that cannot be NULL, while a unique key allows NULL values (usually one) and can be used for alternate keys.

This question belongs to: Computer Database Management System (DBMS)

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Question #1

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What is an attribute called when it can hold more than one value for a single entity instance, such as multiple phone numbers for a person?

A. Derived Attribute

B. Complex Attribute

C. Multi-valued Attribute

D. Composite Attribute

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Correct Answer: Option C

Explanation:
A Multi-valued Attribute is an attribute that can store multiple distinct values for a single entity instance (e.g., skill_set, phone_numbers).

This question belongs to: Computer Database Management System (DBMS)

Question #2

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What type of entity cannot be uniquely identified by its own attributes alone and must rely on a relationship with a parent entity?

A. Abstract Entity

B. Strong Entity

C. Associative Entity

D. Weak Entity

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Correct Answer: Option D

Explanation:
A Weak Entity is an entity that does not possess its own primary key and depends on an identifying relationship with a strong (owner) entity.

This question belongs to: Computer Database Management System (DBMS)