How can a restriction be imposed on the value of a field in an SQL database table?

In SQL, constraints are used to impose restrictions on the values of a field in a table. The main types of constraints that can be applied to a field:

• CHECK — sets a condition that the field's values must satisfy.
• NOT NULL — prohibits storing NULL in the field.
• UNIQUE — guarantees the uniqueness of the value.
• PRIMARY KEY — a unique identifier for a record.

For example, to restrict the age field to values from 18 to 65, you can use a CHECK constraint:

CREATE TABLE employees (
    id INT PRIMARY KEY,
    name VARCHAR(100),
    age INT CHECK (age >= 18 AND age <= 65)
);

If the table is already created, you can add a constraint using ALTER TABLE:

ALTER TABLE employees
ADD CONSTRAINT chk_age CHECK (age >= 18 AND age <= 65);