Oracle 은 어떻게 데이터베이스 에 제약 과정 분석 을 추가 합 니까?

create table stu01(
  sid varchar(100),
  sname varchar2(100)
  --constraint PK_stu01_sid primary key (sid)
 )

 --    (  )
  alter table stu01 add constraint PK_stu01_sid primary key(sid)  ---      
  alter table stu01 drop constraint PK_stu01_sid          ---      

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)
 )

 --     
 create table course01(
  cid varchar2(100),
  cname varchar2(100) not null,
  constraint PK_course01_cid primary key(cid),
  constraint UK_course01_cname unique(cname)
 )

 --       ，           
  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)
  )

 --      
 alter table mark01 drop constraint FK_mark02_sid
  alter table mark01 drop constraint FK_mark02_cid