Mysql创建外键约束的两种方式

create table books(
 bookid number(10) not null primary key,
 bookName varchar2(20) not null,
 price number(10,2),
 categoryId number(10) not null references Category(id) --外键约束
);

create table books(
 bookid number(10) not null primary key,
 bookName varchar2(20) not null,
 price number(10,2),
 categoryId number(10) not null
);
ALTER TABLE books ADD CONSTRAINT FK_Book_categoryid FOREIGN KEY(categoryId ) REFERENCES Category(id);