Candidate Key in DBMS

In a database, the candidate key is the most essential aspect that uniquely identifies every individual record in the table. There can be multiple candidate keys for each table, but out of all, only one candidate key will be designated as the primary key, and the remaining candidate keys (alternate keys) will be available in the context of the table. A well-defined candidate key helps to keep the data accurate, prevents duplicates, and builds strong links between data. In this blog, you will look into what a candidate key is, its types, properties, and why it is a major component of DBMS.

Table of  Contents:

• What is a Candidate Key in DBMS?
• Types of Candidate Keys in DBMS
• Properties of Candidate Key in DBMS
• How to Create a Candidate Key in DBMS
• How to Identify a Candidate Key in DBMS
• Difference Between Candidate Key and Primary Key in DBMS
• Common Mistakes While Using Candidate Key in DBMS
• Best Practices for Using Candidate Key in DBMS
• Conclusion

What is a Candidate Key in DBMS?

A candidate key in a DBMS is a column or a group of columns that can uniquely identify each row in a table. It ensures that no two rows have the same values in those columns. A table can have more than one candidate key. One of them is chosen as the primary key, and the rest are called alternate keys. All candidate keys are valid options for the primary key because they meet the rules for identifying rows. Candidate keys play an important role in database design because they affect how data is stored, searched, and linked to other tables.

Importance of Candidate Key in DBMS:

Candidate keys are a key element of database design. Here is why they are important: 

• Unique: Candidate keys ensure all records are unique with values that are not the same; this will also eliminate the possibility of duplicates in the table. 
• Data Integrity: Candidate keys make things cleaner and more accurate when organizing the data by not incorrectly duplicating data or making mistakes.
• Relationships: Candidate keys relate tables to other tables. Each candidate key in one table becomes a reference in another table. 
• Easy Access: When a candidate key is defined, data retrieved from the table becomes easier to locate. Keys help find rows faster when searching in a table.
• Performance Improvement: Candidate keys help optimize the table structure for returning data in user queries by eliminating duplicates. This makes query processing and data retrieval faster and more efficient.

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Types of Candidate Keys in DBMS

Depending on the number of columns, the candidate key can be categorized into two types:

1. Single-Attribute Candidate Key

A single-attribute candidate key refers to the candidate key that uses only one column or field in a table to create a unique identifier for each record. This means that this column’s value must be unique for every row, and no two rows have the same value in this column. This is the simplest and most reliable type of candidate key. It is used when one field, like an ID or employee code, can uniquely identify each record in the table.

Example: Let’s create a Students table where each student has a Roll Number that is unique. This roll number will act as a single-attribute candidate key. 

-- Creating a table
CREATE TABLE Students (
    RollNumber INT PRIMARY KEY,
    Name VARCHAR(50),
    Age INT,
    Class VARCHAR(20)

);

-- Inserting the data
INSERT INTO Students (RollNumber, Name, Age, Class) VALUES
(101, 'Amit Sharma', 17, '12A'),
(102, 'Sneha Verma', 16, '11B'),
(103, 'Ravi Kumar', 17, '12A'),
(104, 'Pooja Singh', 15, '10C');
-- Displaying the table
SELECT * FROM Students;

Output:

Explanation: Here, this query is used to create a Students table where RollNumber is a single-attribute candidate key

2. Composite Candidate Key

A composite candidate key is used to combine two or more columns in a table to create a unique identity for each row. This happens in a situation where no single column can uniquely identify a row, but, in some circumstances, a combination of columns makes them uniquely identifiable. Each combination must be unique, and there are no two rows that can have the same combination. Composite candidate keys are significant in many real-world scenarios that require relationships of multiple columns to uniquely identify records.

Example: Let’s create a CourseEnrollment table. Each record is used to store which student is enrolled in which course. Here, neither StudentID nor CourseID is unique, but the combination of both should be unique, which makes them a composite candidate key.

-- Creating a table
CREATE TABLE CourseEnrollment (
    StudentID INT,
    CourseID INT,
    EnrollmentDate DATE,
    PRIMARY KEY (StudentID, CourseID)

);

-- Inserting the data into the table
INSERT INTO CourseEnrollment (StudentID, CourseID, EnrollmentDate) VALUES
(101, 501, '2025-07-01'),
(101, 502, '2025-07-02'),
(102, 501, '2025-07-01'),
(103, 503, '2025-07-03');
SELECT * FROM CourseEnrollment ;

Output:

Explanation: Here, this query creates a CourseEnrollment table with a composite primary key( StudentID, CourseID).

3. Alternate Key (Secondary Candidate Key)

When a table has multiple columns that can uniquely identify every row, those columns are referred to as candidate keys. Only one of them is selected as the primary key. The other candidate keys are known as alternate keys. While alternate keys are not the primary key, they are still unique and valid keys. They can be used for search, filtering, and data integrity purposes.

Example: 

-- Creating a table with both a primary key and an alternate key
CREATE TABLE Employees (
    EmployeeID INT PRIMARY KEY,
    Name VARCHAR(50),
    Email VARCHAR(100) UNIQUE,
    Department VARCHAR(50)

);
-- Inserting employee records
INSERT INTO Employees (EmployeeID, Name, Email, Department) VALUES
(1, 'Rahul Mehta', '[email protected]', 'HR'),
(2, 'Anita Roy', '[email protected]', 'Finance'),
(3, 'Vikas Singh', '[email protected]', 'IT'),
(4, 'Sneha Jain', '[email protected]', 'Marketing');
-- Viewing the table data
SELECT * FROM Employees;

Output:

Explanation: Here, this query creates an Employees table having EmployeeID as the primary key and Email as a unique alternate key.

Properties of Candidate Key in DBMS

A candidate key must follow some rules. These rules ensure that the key operates properly in the database.

1. Uniqueness: Every value in the candidate key must be different. No two rows should have the same key value.

2. Minimality: The candidate key includes only the columns needed to identify a row. If any column is removed, it can no longer work as a key.

3. No Null Values: A candidate key cannot have blank or null values. Every row must have a value in the key column.

4. Stability: The values in the candidate key should stay the same. Changing them often can make it harder to manage the data.

5. Non-Redundant: The key should not have extra columns. Each column in the key must help identify the row clearly.

How to Create a Candidate Key in DBMS

Let’s create a table called Employees for a better understanding of the creation of a candidate key in SQL

We will assume that:

• Each employee has a unique ID.
• Each employee also has a unique email.
• Names and departments can be repeated.

Step 1: Create the Table

Create the table and define candidate keys using PRIMARY KEY and UNIQUE constraints.

CREATE TABLE Employees (
    EmployeeID INT PRIMARY KEY,         -- Candidate key selected as Primary Key
    Email VARCHAR(100) UNIQUE,          -- Another candidate key defined using UNIQUE
    Name VARCHAR(50),
    Department VARCHAR(50)
);

Step 2: Inserting the Data into the Table

INSERT INTO Employees (EmployeeID, Email, Name, Department) VALUES
(101, '[email protected]', 'Yash', 'HR'),
(102, '[email protected]', 'Lithin', 'Finance'),
(103, '[email protected]', 'Lokie', 'HR');

Step 3: Display the Result

SELECT * FROM Employees;

Output:

This is how the table looks after creating and inserting the data into it.

How to Identify a Candidate Key in DBMS

• Uniqueness: The column(s) (or combination of columns) must have distinct values for each row. Each row should have different values in the specified column(s).
• Non-null Values: The column(s) cannot have NULLs. Each row must have a value for the specified column(s). 
• Minimality: The candidate key must be minimal by using the least number of columns needed to uniquely identify a row. The candidate key cannot have extra columns.

Note: Let’s understand with the help of the Employees table, which we have created above.

In the above-mentioned Employees table:

1. EmployeeID

EmployeeID is considered a candidate key as:

• It has unique values.
• It has no nulls.
• It can identify each row individually.
• It is only one column, so it’s minimal.

2. Email

Email is considered a candidate key as:

• It has unique values.
• It has no nulls.
• It can identify each row individually.
• It is only one column.

3. Name

Name is not considered a candidate key as:

• It could be repeated.
• It does not identify every row uniquely.

4. Department

Department is not considered a candidate key as:

• It can also be duplicated.
• It does not identify rows uniquely.

Difference Between Candidate Key and Primary Key in DBMS

Let’s look at the difference between a candidate key and a primary key in a DBMS

Common Mistakes While Using Candidate Key in DBMS

Let’s look at some mistakes that users make while working with the candidate key:

1. Assuming Uniqueness Without Proof: Developers sometimes pick columns like Email or Name as keys without checking the data. But these can have duplicate values, which causes problems later.

2. Allowing NULLs in Keys: A candidate key should never have NULL values. Using only UNIQUE is not enough; you also need NOT NULL to make it valid.

3. Using Changeable Columns as Keys: Columns like Email or Phone can change over time. If they are used as keys, updates become hard and risky.

4. Making Keys with Too Many Columns: Some people combine two or more columns to make a key, even when one column is enough. This makes the table harder to manage.

5. Not Testing Data Before Adding Key Constraints: If you don’t check your data first, adding a key constraint can fail. There may already be duplicates or NULLs in the data.

Best Practices for Using Candidate Key in DBMS

Let’s look at some best practices for using a candidate key in DBMS:

1. Always define at Least One Candidate Key: Every table should have at least one column that uniquely identifies each row. This helps keep the data accurate and reliable.

2. Combine UNIQUE and NOT NULL: For a column to be a candidate key, both conditions must be met. You can combine both by simply using the primary key.

3. Select Simple and Stable Columns: Use columns like IDs that do not change over time. Avoid using columns like Email or Name since they can be updated.

4. Use the Smallest Key Possible: If one column is enough to identify rows, use only that. Use multiple columns only if necessary.

5. Clearly Mark Your Keys: Always show which columns are candidate keys in your design or documentation. This helps others understand the structure easily.

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Conclusion

It is important to understand and use candidate keys correctly, as they are essential for designing, building reliable and organized databases. By avoiding common mistakes like allowing NULL values or using columns that change often, and by following key practices like using proper constraints and simple, clear definitions, candidate keys help keep data unique and reliable. They play a key role in identifying exact records and are vital for maintaining data integrity. A strong database design always starts with choosing the right candidate keys.

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Candidate Key in DBMS – FAQs