create table student(  
sno varchar2(10) primary key,  
sname varchar(20),  
sage number(2),  
ssex varchar(5)  
);  

create table teacher(  
tno varchar(10) primary key,  
tname varchar(20)  
);  

create table course(  
cno varchar(10),  
cname varchar(20),  
tno varchar(20),  
constraint pk_course primary key (cno,tno)  
);  

create table score(  
sno varchar(10),  
cno varchar(10),  
score number(4,2),  
constraint pk_sc primary key (sno,cno)  
);