How can restrictions be imposed on a database table?

Constraints in a database table help ensure data integrity. Main types of constraints:

• PRIMARY KEY — a unique identifier for a row, does not allow NULL.
• FOREIGN KEY — a reference to a key in another table, ensures referential integrity.
• UNIQUE — guarantees the uniqueness of values in a column or set of columns.
• NOT NULL — prohibits NULL values in a column.
• CHECK — a condition that values in a column must satisfy.

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),
    role_id INT,
    FOREIGN KEY (role_id) REFERENCES roles(id)
);

In Java, when working with a database through ORM (e.g., Hibernate), constraints can be set using annotations such as @NotNull, @UniqueConstraint, etc.