Difference between Primary Key and Unique Key in RDBMS

In a Relational Database Management System (RDBMS), the primary key and unique key are both used to ensure the uniqueness of data values in a table, but they have distinct differences:

Primary Key:

• A Primary Key is a column (or a set of columns) that uniquely identifies each row in a table.
• It ensures that no two rows in the table have the same values for the primary key column(s).
• By default, a primary key column cannot contain NULL values.
• Each table can have only one primary key.
• Primary key constraints automatically create a unique index on the primary key column(s).

Unique Key:

• A Unique Key is a constraint that ensures all values in a column (or a set of columns) are unique.
• Unlike a primary key, a unique key column can contain NULL values, but only one NULL is allowed.
• Each table can have multiple unique keys.
• Unique key constraints also create a unique index on the unique key column(s).
• Unique keys can be used to enforce the uniqueness of values that are not intended to be primary keys.

Overall, the primary key uniquely identifies each row in a table and is used for data integrity and relational integrity, while the unique key enforces uniqueness on a column or set of columns and helps in maintaining data consistency.