What is a Candidate Key in SQL?

A Candidate Key in SQL is a column or combination of columns that can uniquely identify each row in a table.

In simple words:

A candidate key is a possible column that can become a primary key.

Why Candidate Key is Important

In a database table:

Multiple columns may uniquely identify records

Among those unique columns:

One is selected as Primary Key
Remaining are Candidate Keys

Real-Time Example

Suppose a learning platform stores student details.

Students Table

Student ID	Email	Phone Number	Name
101	naresh@gmail.com	9876543210	Naresh
102	rahul@gmail.com	9876500000	Rahul

Understanding Candidate Keys

In this table:

Student ID is unique
Email is unique
Phone Number is unique

All these columns can uniquely identify rows.

Therefore:

Student ID
Email
Phone Number

are:

Candidate Keys

What Happens Next?

Among candidate keys:

One key is selected as Primary Key

Example:

Student ID -> PRIMARY KEY

Remaining Candidate Keys

Email

Phone Number

usually become:

UNIQUE KEYS

Candidate Key Architecture

Table
   |
--------------------------------------
|          |            |            |
v          v            v            v

StudentID  Email       Phone       Name
   |          |            |
   |          |            |
   ------------ Candidate Keys
               |
               v
       One Selected as
         Primary Key

Characteristics of Candidate Key

Must uniquely identify rows
Cannot contain duplicate values
Should not contain NULL values
Can be single column or multiple columns

Candidate Key vs Primary Key

Feature	Candidate Key	Primary Key
Purpose	Possible unique identifier	Selected unique identifier
Count	Multiple allowed	Only one allowed
NULL Allowed	No	No
Uniqueness	Required	Required

Simple Understanding

All Primary Keys are Candidate Keys.

But:

Not all Candidate Keys become Primary Keys.

Example

Candidate Keys:

Student ID

Email

Phone Number

Chosen Primary Key

Student ID

Other Candidate Keys

Email

Phone Number

become alternate unique identifiers.

How Candidate Keys are Identified

A candidate key must:

Uniquely identify rows
Contain minimal columns

What is Minimal Column?

Minimal means:

No unnecessary columns included.

Bad Example

(Student ID, Name)

Not minimal because:

Student ID

alone is sufficient.

Candidate Key Example in SQL

CREATE TABLE students (

    student_id INT PRIMARY KEY,

    email VARCHAR(100) UNIQUE,

    phone VARCHAR(15) UNIQUE,

    name VARCHAR(100)

);

Explanation

Column	Type
student_id	Primary Key
email	Candidate Key
phone	Candidate Key

What is Alternate Key?

Candidate keys that are not selected as primary key are called:

Alternate Keys

Example

If:

student_id

is primary key:

then:

email

phone

become:

Alternate Keys

Candidate Key vs Unique Key

Feature	Candidate Key	Unique Key
Concept	Logical database design	Database constraint
Uniqueness	Required	Required
NULL Allowed	No	Usually one NULL
Purpose	Possible primary key	Prevent duplicates

Composite Candidate Key

A candidate key can use multiple columns together.

Example

(student_id, course_id)

can uniquely identify enrollment records.

Composite Candidate Key Example

CREATE TABLE enrollments (

    student_id INT,

    course_id INT,

    grade VARCHAR(10),

    UNIQUE(student_id, course_id)

);

Why Composite Candidate Key is Used

Prevents:

Same student enrolling in same course multiple times

Candidate Key Query Flow

Insert Data
      |
      v
Candidate Key Validation
      |
      v
Duplicate Check
      |
      v
Data Stored

Real-Time Banking Example

Banking systems may use:

Account Number
Email
PAN Number

as candidate keys.

Real-Time E-Commerce Example

E-commerce systems may use:

Customer ID
Email
Phone Number

as candidate keys.

Real-Time Hospital Example

Hospital systems may use:

Patient ID
Insurance Number
Email

as candidate keys.

Advantages of Candidate Key

Ensures uniqueness
Improves database design
Supports normalization
Prevents duplicate records

Challenges of Candidate Key

Choosing wrong candidate key
Large composite keys affect performance
Frequent updates create issues

Best Practices

Choose stable columns
Avoid frequently changing values
Prefer numeric primary keys
Use business identifiers as alternate keys

Candidate Key in Normalization

Candidate keys play an important role in:

Normalization
Relationship design
Database optimization

Candidate Key in Microservices

In microservices architecture:

Each service database may define candidate keys independently

Example

User Service
     |
     v
email

Payment Service
     |
     v
transaction_id

Professional Interview Answer

A Candidate Key in SQL is a column or combination of columns that can uniquely identify each row in a table. A table may contain multiple candidate keys, but only one candidate key is selected as the primary key. Candidate keys must contain unique and non-null values and should use minimal columns required for uniqueness. Candidate keys are important in database design, normalization, and maintaining data integrity.

Why Interviewers Like This Answer

Clearly explains uniqueness concept
Compares candidate key with primary key
Includes real-world examples
Shows database design understanding
Covers normalization concepts

Frequently Asked Questions