DROP TABLE STUDY;
DROP TABLE STUDENT;
DROP TABLE COURSE;
/
CREATE TABLE  STUDENT
(
   SID VARCHAR(10),
   SName varchar(30),
   primary key (SID)
);
/
CREATE TABLE COURSE
(
  CID VARCHAR(10),
  CName varchar(50),
  primary key (CID)
);
/

CREATE TABLE STUDY
(
  SID  VARCHAR(10),
  CID  VARCHAR(10),
  SCORE INTEGER  DEFAULT 0 CHECK (SCORE between 0 and 100),
  constraint PK_STUDY  PRIMARY KEY (SID)  --注意此处设置主键约束名,便于后面修改表的主键
);
/
COMMIT;
/

select * from student;
select * from study;
/
--以表 student,study,score 为例
--1.增加列
alter table student  add (sex varchar2(2) DEFAULT '女',age integer,address varchar(100)) ;
/
--2.修改列定义
ALTER TABLE student MODIFY sex VARCHAR2(4)  DEFAULT '男';
/
--3.删除列
ALTER TABLE student DROP COLUMN address;
/
--4.修改列名
ALTER TABLE student RENAME COLUMN   sid to sno;
/
ALTER TABLE student RENAME COLUMN   sno to sid;
/
--5.修改表名
RENAME study TO Learn;
/
RENAME Learn TO study;
/
--6.增加注释
-- 给表添加释
COMMENT ON TABLE STUDY IS '学习信息表';
/
--给列添加注释
COMMENT ON COLUMN STUDY.SID IS '学员编号';
/
--7.增加外键关联
ALTER TABLE study ADD CONSTRAINT FK_study_R_student_S FOREIGN KEY (SID) REFERENCES  student(SID);

/
--8.修改主键
--查找主键约束名
select T.constraint_name from USER_CONSTRAINTS T where table_name='STUDY' and constraint_type='P' AND ROWNUM<2;
/
--删除主键约束
ALTER TABLE study DROP CONSTRAINT  PK_STUDY 
/  
--添加主键
alter table study add constraint PK_STUDY  primary key (cid,sid); 
/
commit;
/