Primary Key in Oracle vs. Unique Key in Oracle — What's the Difference?

In Oracle databases, a Primary Key is a column or a set of columns that uniquely identifies each row in a table. This key is crucial for ensuring data integrity and is often used in database design to establish relationships between tables. On the other hand, a Unique Key in Oracle also provides a guarantee of uniqueness for each row. However, unlike a Primary Key, a Unique Key does not necessarily have to be the main identifier for a row and can be used for additional constraints to maintain data integrity.

The Primary Key in Oracle is a fundamental aspect of relational database design. It not only uniquely identifies each row but also enforces entity integrity by ensuring that no two rows can have the same Primary Key value. Conversely, a Unique Key in Oracle serves a slightly different purpose. While it also ensures that no two rows have the same value in the Unique Key column(s), it allows for the presence of null values, which is not permissible in a Primary Key.

In terms of database indexing, a Primary Key in Oracle automatically creates a unique index on the primary key column(s) to efficiently manage data retrieval and integrity. This indexing is integral to the performance of database operations. In contrast, a Unique Key in Oracle can also have an index, but it is not automatically created as it is with a Primary Key. This means that while both keys enforce uniqueness, their impact on database performance and indexing differs.

Another aspect to consider is the usage in foreign key constraints. In Oracle, a Primary Key is often used as a reference for Foreign Keys in other tables, establishing a strong referential integrity across the database. The Unique Key, while also capable of being referenced by a Foreign Key, is less commonly used for this purpose, as it allows null values which can complicate referential integrity.

Lastly, a Primary Key in an Oracle database is inherently unique and not null, making it the cornerstone of table design and data relationships. The Unique Key, offering more flexibility, is suitable for additional unique constraints where null values are acceptable. Both keys play vital roles in ensuring data accuracy and integrity, but they serve slightly different purposes in database design and operation.