问如何在mysql中向表中添加外键

EN

Cannot add foreign key constraint

create table product (
  pid int NOT NULL,
  name varchar(10),
    PRIMARY KEY (pid)
);

create table trans (
  tid int NOT NULL ,
  productId int NOT NULL,
  userId int NOT NULL,
  PRIMARY KEY (tid),
  FOREIGN KEY (productId) REFERENCES product(pid),
  FOREIGN KEY (userId) REFERENCES user1(uid)
);

create table user1 (
  uid int NOT NULL ,
  location varchar(22),
  PRIMARY KEY (uid)
);