--    列级，表级建立主键
1.create table constraint_test
( name_id number not null constraint cons_name_id primary key,
 old number )
2.create table constraint_test
( name_id number  primary key,
 old number )


drop table constraint_test;
3.create table constraint_test
( 
name_id number not null,
old number  ,
constraint cons_name_id primary key ( name_id ) 
);


drop table constraint_test;


4.create table constraint_test
( 
name_id number not null,
old number   
);
alter table constraint_test add constraint cons_name_id primary key ( name_id );

drop table course ;
drop table students ;
create table students
(code number  ,
name varchar2(10),
country varchar2(30)
);


alter table students add constraint pk_st_cod primary key ( code);
insert into students values(0001,‘zhangsan‘,‘shanghai‘);
insert into students values(0002,‘lisi‘,‘beijing‘);
insert into students values(0003,‘wangwu‘,‘guangzhou‘);


create table course 
(id number,
code  number,
name varchar2(10),
subject varchar2(30)
);
alter table course  add constraint pk_co_id primary key ( id);
alter table course  add constraint fk_co_st foreign key ( code) references  students(code);
alter table students add foreign key pk_co_id on id;
inser into course(id,code,name,subject) values(1,001,‘zhangsan‘,‘yuwen‘);
inser into course(id,code,name,subject) values(2,001,‘zhangsan‘,‘shuxue‘);