新建一张学员信息表(student)，要求：
1.字段如下：学号(sid)，姓名(name)，性别(sex)，年龄(age)，地址(address)．
2.分别为字段添加约束：学号为主键，姓名为非空，性别为检查约束，年龄为检查约束，地址为默认约束．
SQL>create table student(
sid int constraint student_sid_pk primary key,
name varchar2(20) constraint student_name_nn not null,
sex varchar(20) constraint student_sex_ck check（sex in （'man', 'woman'））,
age int constraint student_age_ck check(age>0),
address varchar2(20) default 'sdzb');
3.创建序列插入学号（sid）列建议初始值从为1001,增量为1.
SQL> create sequence test_sid increment by 1 start with 1001;
4.插入记录．
SQL> insert into student values (test_sid.nextval, 'Li', 'man', 20, 'sdwf');
SQL> insert into student values (test_sid.nextval, 'Xu', 'man', 23, 'sdgm');
SQL> insert into student values (test_sid.nextval, 'zhao', 'woman', 25, 'sdwf');
SQL> insert into student values (test_sid.nextval, 'Si', 'woman', 24, default);
新建一张课程表(course)，要求：
1.字段如下：课程编号(cid)，课程名称(subject)．
2.分别为字段添加约束：课程编号为主键，课程名称为非空．
SQL> create table course (
cid int constraint course_cid_pk primary key,
subject varchar2(20) constraint course_subject_nn not null);
3.创建序列插入课程编号列(cid),建议初始值从为1,增量为1.
SQL> create sequence test_cid increment by 1 start with 1;
4.插入记录．
SQL> insert into course values (test_cid.nextval,'oracle');
SQL> insert into course values (test_cid.nextval,'English');
SQL> insert into course values (test_cid.nextval,'Java');
SQL> insert into course values (test_cid.nextval,'SQL');
新建一张学员考试成绩表(grade)，要求：
1.字段如下：成绩编号(gid)，学号(sid)，课程编号(cid)，考试成绩(score).
2.分别为字段添加约束：成绩编号为主键，学号为外键，课程编号为外键，考试成绩为非空.
SQL> create table grade (
gid int constraint grade_gid_pk primary key,
sid int constraint grade_sid_fk references student(sid),
cid int constraint grade_cid_fk references course(cid),
score int constraint grade_score_nn not null);
3.创建序列插入成绩编号列(gid),建议初始值从为101,增量为1.
SQL> create sequence test_gid increment by 1 start with 101;
4.插入记录．
SQL> insert into grade values (test_gid.nextval,1001,1,60);
SQL> insert into grade values (test_gid.nextval,1001,1,70);
SQL> insert into grade values (test_gid.nextval,1001,1,40);
SQL> insert into grade values (test_gid.nextval,1001,2,80);
SQL> insert into grade values (test_gid.nextval,1001,2,90);
SQL> insert into grade values (test_gid.nextval,1001,3,60);
SQL> insert into grade values (test_gid.nextval,1002,1,65);
针对以上三张表，要求完成如下：
1.按照课程编号分组并求出每一组的平均分数(每门课程的平均分)
SQL> select cid, avg(score) from grade group by cid;
2.按照课程编号分组并求出每一组及格人数(grade>=60)的平均分数(每门课程的及格人数的平均分)
SQL> select cid, avg(score) from grade where score>60 group by cid;
3.求每个学员所有的课程的平均分
SQL> select sid, avg(score) from grade group by sid;
4.求每个学员所有的（考试成绩）及格课程的平均分
SQL> select sid, avg(score) from grade where score>60;
5.每次内部测试不同学员的平均成绩(每个学员的每门课程的平均分)
SQL> select sid, cid, avg(score) from grade group by sid, cid;
6.查询补考过的学员的平均成绩(求出学员的课程编号(cid)在分组内出现过一次以上)(每个学员的每门课
程的课程编号出现过一次以上)
SQL> select sid, cid, avg(score) from grade group by sid,cid having count (cid)>1;
7.使用多表内连接查询，求出学员的学号，姓名，考号，科目和成绩
SQL> select s.sid, s.name, g.gid, c.subject, g.score
from student s, course c, grade g
where s.sid=g.sid
and g.cid=c.cid;
8.建立一张视图，包含学员的学号，姓名，考号，科目，成绩这些字段
SQL> create view vi_t1 as
select s.sid, s.name, g.gid, c.subject, g.score
from student s, course c, grade g
where s.sid=g.sid
and g.cid=c.cid;