Differentiate between Candidate Key and Primary Key in the context of Relational Database Model. OR Consider t

Question 78 of 95

Q. Differentiate between Candidate Key and Primary Key in the context of Relational Database Model.

(OR)

Consider the following table PLAYER: Table: PLAYER PNO NAME SCORE P1 RISHABH 52 P2 HUSSAIN 45 P3 ARNOLD 23 P4 ARNAV 18 P5 GURSHARAN 42

(a) Identify and write the name of the most appropriate column from the given table PLAYER that can be used as a Primary key.

(b) Define the term Degree in relational data model. What is the Degree of the given table PLAYER?

Puducherry TnboardCBSE Class XII Board 2022Subjective· 2mImportance★★★★★

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Part (a)Concept understanding — Primary Key Identification

Primary Key Identification

Think about how you tell two students apart in a classroom. If you call out "Rahul," there might be three Rahul's. But if you say "Rahul Sharma, roll number 12," you've pointed to exactly one person. That unique identifier — the roll number — is doing the job of a primary key.

The Everyday Intuition

Every collection of things needs a way to refer to each item without confusion. In a library, every book has a unique accession number. In a bank, every account has an account number. In a school, every student has a unique admission number. These are not random — they are deliberately chosen so that no two items share the same identifier.

This is the core idea: a primary key is a single attribute (or a combination of attributes) that uniquely identifies each record in a table. No two rows can have the same primary key value, and no primary key can be empty (null).

The Precise Meaning

In database terms, a primary key serves two non-negotiable rules:

Uniqueness — every value in the primary key column must be different from every other value. If you have 100 students, you need 100 distinct roll numbers.
Non-null — every record must have a primary key value. You cannot have a student with "no roll number."

A primary key can be a single column, like a student ID. Or it can be a composite key — two or more columns taken together to create uniqueness. For example, in a table recording which books students have borrowed, neither "Student ID" alone nor "Book ID" alone is unique (a student borrows many books; a book is borrowed by many students over time). But the combination of "Student ID + Book ID + Date Borrowed" together can be unique.

Important

A primary key is not just any unique column. It is the official, chosen identifier for that table. A table can have many columns that are unique (like Aadhaar number, PAN number, and email), but only one of them is designated as the primary key. The others are called candidate keys — they could have been the primary key, but weren't chosen.

Why It Matters

Without a primary key, you cannot reliably update or delete a specific record. Imagine a school database where two students have the same name "Ananya Sharma." If you try to delete "Ananya Sharma," the database has no way to know which one you mean. With a primary key, you say "delete student with roll number 45" — and it's unambiguous. …

Part (b)Concept understanding — Primary Key Identification

Primary Key Identification

The Everyday Intuition

The Precise Meaning

In database terms, a primary key serves two non-negotiable rules:

Uniqueness — every value in the primary key column must be different from every other value. If you have 100 students, you need 100 distinct roll numbers.
Non-null — every record must have a primary key value. You cannot have a student with "no roll number."

Important

Why It Matters

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Part (a): a candidate key is any minimal unique identifier (many possible); the primary key is the chosen one and must be non-null.

Part (b): (a) PNO is the primary key of PLAYER; (b) Degree = number of columns = 3.

Part (a)

Candidate Key. A candidate key is an attribute, or a minimal set of attributes, that uniquely identifies each tuple (row) of a relation. "Minimal" means no attribute can be dropped without losing uniqueness. A single table can have more than one candidate key (for example both a Roll Number and an Aadhaar Id could each identify a student).

Primary Key. The primary key is the one candidate key selected by the database designer to be the table's main identifier. Two rules bind it: values must be unique, and — unlike other candidate keys — it can never be NULL (entity integrity). The remaining candidate keys are called alternate keys.

Aspect	Candidate Key	Primary Key
How many	A table may have several	Exactly one per table
Selection	All are potential identifiers	The chosen candidate key

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Q.Differentiate between Candidate Key and Primary Key in the context of Relational Database Model.

Part (a)

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