In a database, a primary key and a unique key are two different types of constraints that help maintain the integrity of the data stored in a table. While both keys are used to ensure that there are no duplicate values in a table, they have different characteristics and serve different purposes. In this article, we will explore the differences between a primary key and a unique key.

Primary Key: A primary key is a unique identifier for each row in a table. It is used to ensure that each row in a table is uniquely identified and that there are no duplicates. The primary key constraint is used to enforce the uniqueness of the primary key column and to ensure that the column does not contain NULL values. A primary key can be defined on one or more columns in a table, but each column in the primary key must be unique.

A primary key is often used to create relationships between tables. For example, if a table has a primary key column that is used as a foreign key in another table, the primary key column can be used to join the two tables. In addition, a primary key is often used to enforce referential integrity between tables. When a primary key is defined on a table, the table cannot contain any foreign keys that reference a value that does not exist in the primary key column of the related table.

Unique Key: A unique key is also used to ensure that there are no duplicate values in a table. However, unlike a primary key, a unique key does not necessarily provide a unique identifier for each row in a table. Instead, a unique key is used to ensure that a particular column or set of columns in a table does not contain duplicate values.

A unique key constraint is used to enforce the uniqueness of the unique key column or columns, but it does not prevent the column from containing NULL values. A unique key can be defined on one or more columns in a table, and each column in the unique key must be unique.

A unique key is often used to enforce business rules or constraints. For example, a table that stores customer data might have a unique key constraint on the email address column to ensure that each customer has a unique email address. This constraint ensures that there are no duplicate email addresses in the table, which can be important for sending emails or contacting customers.

Differences between Primary Key and Unique Key:

Uniqueness: A primary key must be unique for each row in a table, while a unique key ensures that a particular column or set of columns in a table does not contain duplicate values.

NULL Values: A primary key cannot contain NULL values, while a unique key can contain NULL values.

Relationships: A primary key is often used to create relationships between tables, while a unique key is often used to enforce business rules or constraints.

Limitations: A table can have only one primary key, while a table can have multiple unique keys.

Indexing: A primary key is automatically indexed by the database management system, while a unique key may or may not be indexed.

In conclusion, while both primary keys and unique keys are used to ensure data integrity in a table, they have different characteristics and serve different purposes. A primary key is used to uniquely identify each row in a table and enforce relationships and referential integrity, while a unique key is used to enforce business rules and ensure that a particular column or set of columns in a table does not contain duplicate values. Understanding the differences between primary keys and unique keys is important for designing and maintaining effective database systems.