求大侠帮忙！急急急！
--添加约束（sno 为主键 sex的值只能为男和女,age的范围是16-28）
create table student (
sno char(6) primary key,
sname varchar2(10),
sex char(2),
age number(3),
dept varchar2(10)
);
alter table student
add constraint CK_age check(age between 16 and 28);

alter table student
add constraint CK_sex check(sex in('男','女'));


--课程表
create table course (
cno char(5) primary key,
cname varchar2(12),
ccredit number ---学分
);


--选课表
--（添加约束 sno cno 为联合主键 同时还是外键）
create table sc(
sno char(6),
cno char(5),
score number(3)
primary key (sno,cno));
alter table sc
add constraint FK_sno foreign key(sno) references student(sno);


sno,cno,同时还为外键不知道怎么写。求帮忙！！！！！！
[最优解释]

SQL> alter table sc add constraint sc_fk1 foreign key(sno) references
  2  student(sno) on delete cascade;

Table altered.

SQL> alter table sc add constraint sc_fk2 foreign key(cno) references
  2  course(cno) on delete cascade;

Table altered.