What is a primary key and what are its mandatory constraints?

Primary Key is a unique identifier of a record in a database table. It ensures the uniqueness of each row and is used for quick search and linking with other tables.

Mandatory constraints of the primary key:

• Uniqueness: values in the primary key column(s) must be unique to unambiguously identify a record.
• NOT NULL: values cannot be NULL, as the key must uniquely identify a record.

Example in SQL:

CREATE TABLE Users (
  id INT PRIMARY KEY, -- unique and NOT NULL
  name VARCHAR(100)
);