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

Main characteristics of a primary key in a database:

• Uniqueness: The primary key values must be unique for each record in the table. This guarantees unambiguous identification of each row.

• Cannot be NULL: The field(s) used in the primary key cannot contain NULL, as this would violate uniqueness and identification.

• Stability: The primary key values should not change frequently, as this can lead to data integrity and relationship issues.

• Single column or composite key: The primary key can consist of one column or multiple columns (composite key).

• Indexing: Usually, the database automatically creates an index on the primary key to speed up searches.

Example of creating a table with a primary key in SQL:

CREATE TABLE Users (
    id INT PRIMARY KEY,
    username VARCHAR(50),
    email VARCHAR(100)
);

Here, id is the primary key, unique and cannot be NULL.