MySQL Primary Key vs Candidate Key

The context of relational databases, a “candidate key” is a broader concept compared to a “primary key.” Both are used to uniquely identify records in a table and enforce data integrity, but there are important differences between them.

Candidate Key:

1. Definition:

• A candidate key is a column or a set of columns that could be used as the primary key of a table.
• It satisfies the uniqueness and non-null requirements, just like a primary key.

1. Uniqueness:

• Like a primary key, a candidate key enforces the uniqueness constraint.
• There can be multiple candidate keys in a table.

1. Nullability:

• Candidate key columns cannot contain NULL values, just like primary keys.

1. Role:

• A candidate key serves as a potential primary key option for a table.
• Not all candidate keys become primary keys, as only one is chosen as the primary key.

1. Indexing:

• Like a primary key, a candidate key can automatically create an index on the table to enforce uniqueness and improve performance.

Primary Key:

1. Definition:

• A primary key is a specific candidate key that is chosen to uniquely identify records in a table.
• It is selected as the main identifier for the table.

1. Uniqueness:

• A primary key enforces the uniqueness constraint just like a candidate key.

1. Nullability:

• Primary key columns cannot contain NULL values, just like candidate keys.

1. Role:

• The primary key serves as the main identifier for the table.
• It is used as a reference in foreign key relationships to establish relationships between tables.

1. Indexing:

• A primary key automatically creates an index on the table, often optimizing retrieval and search operations.

Usage:

• Use candidate keys to identify all the possible options for primary keys in a table.
• Choose one of the candidate keys as the primary key to be the main identifier for the table.

Example:

Consider a Students table with columns student_id, student_email, and student_roll. Both student_id and student_email could be potential candidate keys, as they are unique and non-null. However, you might choose student_id as the primary key for this table.

-- Candidate Keys
ALTER TABLE Students
ADD CONSTRAINT unique_email UNIQUE (student_email),
ADD CONSTRAINT unique_roll UNIQUE (student_roll);

-- Primary Key
ALTER TABLE Students
ADD PRIMARY KEY (student_id);

In summary, a candidate key represents all the possible options for a primary key in a table, while a primary key is the specific candidate key chosen to uniquely identify records and serve as the main identifier.