What is a primary key and what are its constraints?

A Primary Key is a unique identifier for a record in a database table. It guarantees the uniqueness of each row and is used for quick data retrieval and establishing relationships between tables.

Main constraints of a primary key:

• Uniqueness: values in the primary key column(s) must be unique to unambiguously identify a record.
• Cannot be NULL: the primary key must have a value; NULL is not allowed.
• Immutability: although technically possible to change, in practice, primary key values are not changed to maintain data integrity.

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

CREATE TABLE Users (
    UserID INT PRIMARY KEY,
    UserName VARCHAR(100),
    Email VARCHAR(100)
);