Add Foreign Key to existing table

To add a foreign key (grade_id) to an existing table (users), follow the following steps:

ALTER TABLE users ADD grade_id SMALLINT UNSIGNED NOT NULL DEFAULT 0;
ALTER TABLE users ADD CONSTRAINT fk_grade_id FOREIGN KEY (grade_id) REFERENCES grades(id);

Simply use this query, I have tried it as per my scenario and it works well

ALTER TABLE katalog ADD FOREIGN KEY (`Sprache`) REFERENCES Sprache(`ID`);