What methods can be used to impose constraints on an existing table in a database?

To impose constraints on an existing table in a database, the ALTER TABLE command is usually used. It allows adding various constraints, such as:

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

An example of adding a uniqueness constraint:

ALTER TABLE employees
ADD CONSTRAINT unique_email UNIQUE (email);

You can also use migration tools (such as Flyway or Liquibase) to manage database schema changes in your project.