create table stu01(

     sid varchar(100),

     sname varchar2(100)

     --constraint PK_stu01_sid primary key (sid)

   )

   --添加约束(主键)

     alter table stu01 add constraint PK_stu01_sid primary key(sid)   ---添加主键约束

     alter table stu01 drop constraint PK_stu01_sid                   ---删除主键约束

 create table stu02(

     sid varchar2(100),

     sname varchar2(100),

     constraint PK_stu02_sid primary key(sid),

     constraint UK_stu02_sname unique(sname)

   )

   create table stu03(

     sid varchar2(100),

     sname varchar2(100) not null,

     constraint PK_stu03_sid primary key(sid),

     constraint UK_stu03_sname unique(sname)

   )
   

  create table stu04(

     sid varchar2(100),

     sname varchar2(100) not null,

     constraint PK_stu04_sid primary key(sid),

     constraint UK_stu04_sname unique(sname)

   )

   --创建课程表

   create table course01(

     cid varchar2(100),

     cname varchar2(100) not null,

     constraint PK_course01_cid primary key(cid),

     constraint UK_course01_cname unique(cname)

   )

   --最后添加成绩表，外键约束课程表和学生表

    create table mark02(

     sid varchar2(100),

     cid varchar2(100),

     mark number,

     constraint PK_mark02_sidAndcid primary key(sid,cid),

     constraint FK_mark02_sid foreign key(sid) references stu04(sid),

     constraint FK_mark02_cid foreign key(cid) references course01(cid),

     constraint UK_mark02_mark unique(mark)

    )

   --删除外键约束

   alter table mark01 drop constraint FK_mark02_sid

     alter table mark01 drop constraint FK_mark02_cid