Oracle中给表添加主键、外键

1、创建表的同时创建主键约束

（1）无命名 create table student ( studentid int primary key not null, studentname varchar(8), age int);

（2）有命名 create table students ( studentid int , studentname varchar(8), age int, constraint yy primary key(studentid));

2、删除表中已有的主键约束

（1）无命名可用 SELECT * from user_cons_columns; 查找表中主键名称得student表中的主键名为SYS_C002715 alter table student drop constraint SYS_C002715;

（2）有命名 alter table students drop constraint yy;

3、向表中添加主键约束 alter table student add constraint pk_student primary key(studentid);

4、向表中添加外键约束 ALTER TABLE table_A ADD CONSTRAINT FK_name FOREIGN KEY(id) REFERENCES table_B(id);

下面举个创建表的同时创建外键的例子：

两张表 A B
A内有主键ID
B内有外键A_ID
怎么把这两张表创建主外键关系??

create TABLE zhao(
  id number primary key,
  mingcheng nvarchar2(50),
  neirong nvarchar2(50),
  jiezhiriqi date,
  zhuangtai nvarchar2(50)
);
create TABLE tou(
  id number primary key,
  zhao_id number,
  toubiaoqiye nvarchar2(50),
  biaoshuneirong nvarchar2(50),
  toubiaoriqi date,
  baojia number,
  zhuangtai nvarchar2(50),
  foreign KEY(zhao_id) REFERENCES zhao(id)
);

references 不是
references to