Explain what data integrity constraints are in SQL and how they are used.

Data integrity constraints in SQL are rules that ensure the correctness and consistency of data in a database.

Main types of constraints:

• PRIMARY KEY — a unique identifier for a row.
• FOREIGN KEY — a reference to a key from another table, ensuring the relationship and integrity between tables.
• UNIQUE — guarantees the uniqueness of a value in a column.
• NOT NULL — prohibits storing empty values.
• CHECK — verifies a condition for a value.

Example of using a constraint:

CREATE TABLE users (
    id INT PRIMARY KEY,
    email VARCHAR(255) UNIQUE NOT NULL,
    age INT CHECK (age >= 18)
);

Constraints help prevent errors, such as data duplication or mismatch of relationships between tables.