What are the main characteristics that a primary key in a database can have?

Main characteristics of a primary key in a database:

• Uniqueness: each primary key value must be unique to unambiguously identify a record.
• Cannot be NULL: the primary key must have a value; NULL is not allowed.
• Immutability: primary key values usually do not change after record creation.
• Simplicity: it is recommended to use a simple data type (e.g., numeric or string identifier).
• Indexability: an index is automatically created on the primary key for fast searching.

Example in SQL:

CREATE TABLE users (
  id INT PRIMARY KEY,
  name VARCHAR(100)
);