怎么样在oracle中添加删除主键？

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

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

 alter table students drop constraint yy;

SELECT * from user_cons_columns;

alter table student drop constraint SYS_C002715;

alter table student add constraint pk_student primary key(studentid);