--创建数据表Student
create table Student(
SID number(2) constraint PK_SID primary key,--指定该列为主键列，并指定主键名为PK_SID
SName varchar2(16) not null
)--创建数据表Class
create table Class(
CID number(2) constraint PK_CID primary key,--指定该列为主键列，并指定主键名为PK_CID
CName varchar2(16) not null
)

--将数据表Student重命名为Stu
alter table Student rename to Stu;--删除数据表Studentdrop table Student;

--为数据表Student添加字段SGender和SCID
alter table Student add (SGender char(2));
alter table Student add (SCID number(2));
--删除数据表Student中的字段SGender
alter table Student drop column SGender;
--将数据表Student中的字段SID重命名为StuID
alter table Student rename column SID to StuID;
--修改数据表Student中字段SID的数据类型
alter table Student modify SID number(2);

--为数据表Student中的字段SGender添加约束，并指定该约束的名称为ch_gender，指定该列的值只能是'男'或'女'
alter table Student add constraint ch_gender check(SGender='男' or SGender='女');
--删除数据表Student中约束名为ch_gender的约束
alter table Student drop constraint ch_gender;

--查看数据表Student中已定义的主键
select * from user_cons_columns where table_name='STUDENT';
--将数据表Student中的字段SName设为主键列，并指定该主键的名称为PK_Name
alter table Student add constraint PK_Name primary key(SName);
--删除主键名为PK_Name的主键
alter table Student drop constraint PK_Name;
--将主键名PK_StuID重命名为PK_SID
alter table Student rename constraint PK_StuID to PK_SID;
--禁用主键
alter table Student disable primary key;
--启用主键
alter table Student enable primary key;

--查看数据表中已存在的外键
select owner,constraint_name from user_constraints where constraint_type='R'--P 主键 R 外键
--添加外键
alter table Student add constraint FK_SCID foreign key(SCID) references Class(CID)
--删除外键
alter table Student drop constraint FK_SCID
--重命名外键
alter table Student rename constraint FK_SClassID to FK_SCID
--禁用外键
alter table Student disable constraint FK_SCID
--启用外键
alter table Student enable constraint FK_SCID