轉自：https://blog.csdn.net/baidu_37107022/article/details/74853467

1、建立表時直接建立外來鍵約束

create table books(
    bookid number(10) not null primary key,
    bookName varchar2(20) not null,
    price number(10,2),
    categoryId number(10) not null references Category(id)  --外來鍵約束
);

備註：必須先建立參照表，才能在建立外來鍵約束，即必須現有表Category，再有book

2、先建立表，表建立成功後，單獨新增外來鍵約束

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);