How can a restriction be implemented in an existing database table?

To add a constraint to an existing database table, the ALTER TABLE command is usually used. Depending on the type of constraint, the syntax may vary. For example, to add a uniqueness constraint:

ALTER TABLE table_name
ADD CONSTRAINT constraint_name UNIQUE (column_name);

To add a foreign key:

ALTER TABLE table_name
ADD CONSTRAINT fk_constraint_name FOREIGN KEY (column_name) REFERENCES other_table(other_column);

It is important to note that some DBMSs may have restrictions on altering tables with data, and it may be necessary to verify existing data for compliance with the new constraint beforehand.