MySQL Primary Key vs Unique key

Both primary keys and unique keys in MySQL are used to enforce data integrity by ensuring that certain columns or sets of columns have unique values. However, they have different roles and implications within a database. Let’s compare MySQL primary keys and unique keys:

Primary Key:

1. Uniqueness:

• A primary key is a column or set of columns that uniquely identify each row in a table.
• It ensures that there are no duplicate values in the primary key column(s).

1. Nullability:

• A primary key column cannot contain NULL values.
• It must have a value for each row.

1. Number of Columns:

• A table can have only one primary key.
• If a primary key spans multiple columns, the combination of values must be unique.

1. Role:

• A primary key is used to uniquely identify a record in a table.
• It is a reference point for establishing relationships with foreign keys in related tables.

1. Indexing:

• A primary key automatically creates a clustered index on the table.
• This can optimize retrieval and search operations.

Unique Key:

1. Uniqueness:

• A unique key ensures that the values in the column(s) are unique, just like a primary key.
• Unlike a primary key, a table can have multiple unique keys.

1. Nullability:

• A unique key column can contain NULL values.
• However, only one NULL value is allowed per unique key column.

1. Number of Columns:

• A unique key can consist of one or multiple columns.
• The combination of values must be unique for the unique key.

1. Role:

• A unique key enforces the uniqueness of the values in the specified column(s).
• It can be used to ensure data integrity for columns that need to be unique but don’t necessarily serve as the main identifier.

1. Indexing:

• A unique key automatically creates a non-clustered index on the table.
• This index supports fast retrieval of data and ensures uniqueness.

Usage:

• Use a primary key to uniquely identify records and serve as the main identifier of a table.
• Use a unique key to enforce uniqueness in columns that need to have distinct values but don’t necessarily serve as the primary identifier.

Example:

Let’s consider an Employees table. A primary key on the employee_id column uniquely identifies each employee. Additionally, a unique key on the email column ensures that no two employees share the same email address.

-- Primary Key
ALTER TABLE Employees
ADD PRIMARY KEY (employee_id);

-- Unique Key
ALTER TABLE Employees
ADD CONSTRAINT unique_email UNIQUE (email);

The both primary keys and unique keys are crucial for maintaining data integrity and uniqueness in a relational database. The choice between using a primary key or a unique key depends on the role and purpose of the column(s) you’re enforcing uniqueness on.