What database constraints do you know and use in SQL?

In SQL, there are various constraints that ensure data integrity:

• PRIMARY KEY — a unique identifier for a record, cannot be NULL.
• FOREIGN KEY — a reference to a key from another table, ensures a relationship between tables.
• UNIQUE — guarantees the uniqueness of values in a column.
• NOT NULL — prohibits NULL values in a column.
• CHECK — sets a condition that values in a column must meet.

Example of creating a table with constraints:

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

These constraints help prevent errors and maintain data correctness.