1）创建一个用户名称为user1并授予权限至少能够访问hr.employees（不能授予dba权限）
SYS@ORA11GR2>create user user1 identified by user1;

User created.
SYS@ORA11GR2>grant create session,create table,create sequence,create view to user1;

2）用user1用户创建表students(学号 数值类型 主键约束stud_sno_pk,班级 字符类型外键参照班级表的班级列，姓名 字符类型 非空）;
USER1@ORA11GR2>create table students
  2  (sno number constraint stud_sno_pk primary key,
  3  scno number(20),
  4  sname varchar2(20),
  5  constraint stud_scl_fk foreign key (scno) references classes (cno));

Table created.

3)用user1用户创建表classes(班级 数值类型 主键约束）;
USER1@ORA11GR2>create table classes
  2  (cno number(5) constraint pk_cno primary key using index);

Table created.
4)模拟向以上两个表中插入数据，并且在数据字典中查询两个表的约束信息并显示。
USER1@ORA11GR2>insert into classes values(01);

1 row created.

USER1@ORA11GR2>insert into classes values(02);

1 row created.

USER1@ORA11GR2>insert into students values(1,01,‘zhangsan‘);

1 row created.

USER1@ORA11GR2>insert into students values(2,02,‘lisi‘);

1 row created.

USER1@ORA11GR2>select constraint_name,constraint_type,table_name
  2  from user_constraints
  3  where table_name=upper(‘students‘);
USER1@ORA11GR2>select constraint_name,constraint_type,table_name
  2  from user_constraints
  3  where table_name=upper(‘classes‘);