mysql cannot add foreign key constraint code example

Example 1: alter table add foreign key mysql

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

Example 2: mysql alter add foreign key

ALTER TABLE ordenes ADD ticket VARCHAR(50) NOT NULL;
ALTER TABLE ordenes ADD CONSTRAINT fk_ticket FOREIGN KEY (ticket) REFERENCES tickets(ticket);
// I'm Horrible Hyena

Example 3: mysql alter add foreign key

ALTER TABLE ordenes ADD ticket VARCHAR(50) NOT NULL;
ALTER TABLE ordenes ADD CONSTRAINT fk_ticket FOREIGN KEY (ticket) REFERENCES tickets(ticket);