Kuidas saab kehtestada piiranguid juba olemasolevale tabelile andmebaasis?

To impose constraints on an existing table in a database, the ALTER TABLE command is usually used. With it, you can add various constraints, for example:

• PRIMARY KEY — unique identifier of the record.
• FOREIGN KEY — reference to another table.
• UNIQUE — uniqueness of the value in the column.
• CHECK — condition that the values must satisfy.
• NOT NULL — prohibition of storing NULL.

Example of adding a uniqueness constraint:

ALTER TABLE employees
ADD CONSTRAINT unique_email UNIQUE (email);

You can also use migration tools (e.g., Flyway or Liquibase) to manage changes to the database schema in the project.