Kas oleksite võimeline selgitama, mida SQL-i piirangud tähendavad ja kuidas neid rakendatakse?

Constraints in SQL are rules applied to table columns to ensure data integrity and correctness of information. They help prevent errors and inconsistencies.

Main types of constraints:

• PRIMARY KEY — a unique identifier for the row, cannot be NULL.
• FOREIGN KEY — ensures referential integrity between tables.
• UNIQUE — guarantees the uniqueness of values in the column.
• NOT NULL — prohibits storing empty values.
• CHECK — sets a condition that data 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)
);

Here, we guarantee that each user will have a unique id and email, and that the age will not be less than 18.