What are the differences between a primary key and a unique constraint?

A primary key uniquely identifies each record in a table, while a unique constraint prevents duplicate values in a column or set of columns.

A primary key is a special relational database table column (or combination of columns) designated to uniquely identify all table records. A primary key’s main features are: it must contain a unique value for each row of data, it cannot contain null values, and it must be able to uniquely identify each record in a table. A table can have only one primary key, which may consist of single or multiple fields. When multiple fields are used as a primary key, they are called a composite key.

On the other hand, a unique constraint is a rule that is applied to a column or set of columns in a table to prevent duplicate values. It ensures that all values in a column are different. Unlike primary keys, unique constraints can contain null values, and more than one unique constraint can be added to a table. However, just like primary keys, unique constraints provide a way to enforce the uniqueness of a column or set of columns.

The main difference between a primary key and a unique constraint lies in their purpose and usage. A primary key is used to ensure data in the specific column is accurate and reliable, and uniquely identifies each record in the table. It is a fundamental aspect of database design and plays a crucial role in many types of database operations.

In contrast, a unique constraint is used to ensure that all values in a column or set of columns are unique across the whole table. This can be useful in situations where you want to prevent duplicate entries, such as usernames or email addresses. However, unlike a primary key, a unique constraint does not necessarily identify a single row of data uniquely.

In summary, while both primary keys and unique constraints are used to enforce uniqueness in columns, they serve different purposes and have different rules. A primary key uniquely identifies each record in a table and cannot contain null values, while a unique constraint prevents duplicate values in a column or set of columns and can contain null values.