建表并修改表约束

主键约束 primary key
           not null
           check
           unique 唯一约束          

 create table student( --学生表
           xh number(4) constraint pk_stu primary key, --学号主键
           xm varchar2(10) constraint nn_stu not null, --姓名不能为空
           sex char(2) constraint ck_stu_sex check (sex in ('男','女')), --性别
           birthday date constraint uq_bir unique, --日期
           sal number(7,2) constraint ck_sal check (sal between 500 and 1000)--奖学金 sal >=500 and sal <=1000
        );
      <2>建立约束的同时给约束指定名字，便于删除
        create table cla( --班级表
          id number(2) constraint pk_cla primary key, --班级编号
          cname varchar2(20) constraint nn_cla not null --班级名字
       );
     
      create table stu( --学生表
          xh number(4) constraint pk_stu primary key, --学号是主键
          xm varchar2(20) constraint nn_stu not null, --姓名非空
          age number(2) constraint ck_stu check (age between 10 and 90),--年龄在10到90之间(10<= age  <=90 )
          birthday date,
          shenfenzheng number(18) constraint uq_stu unique, --身份证唯一 
          classid number(2) constraint fk_stu references cla(id) -- 班级编号外键
           --(引用的一定是另外表的主键或唯一性约束的字段)
         );
3）建完表后加约束
加约束
   加主键
    alter table student add constraint pk_stu
    primary key (xh);
   加非空
    alter table student modify (xm not null);
   检查约束
    alter table student add check(sex in ('男','女'));
    alter table student add constraint ck_sal check(sal between 500 and 1000));
添加 主键
 alter table cla add constraint pk_cla
       primary key (id);
加 not null
 alter table cla modify
       (cname not null);

学生表student:

      create table student(
          xh number(4) ,
          xm varchar2(20) ,
          age number(2),
          birthday date,
          shenfenzheng number(18),
          classid number(2)  references cla(id)       
         );

加外键约束
alter table student add constraint fk_stu
    foreign key (classid) references cla(id);

加主键
alter table student add constraint pk_stu
 primary key (xh);

加not null
alter table student modify(xm not null);

加检查约束
alter table student add constraint cc_age
 check (age >= 10 and age <=90);

加唯一约束
  alter table student add constraint
      uq_sfz unique(shenfenzheng);

加外键约束
 alter table student add constraint
    fk_stu foreign key (classid)
     references cla(id);