Answers for "mysql add foreign key into existing table"

alter table add foreign key mysql

ALTER TABLE orders
ADD 
FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE;

fk in insert mysql

INSERT INTO joke(joke_text, joke_date, author_id)
VALUES (‘Humpty Dumpty had a great fall.’, ‘1899–03–13’, 
        (SELECT id FROM author WHERE author_name = ‘Famous Anthony’));