任务要求：
具体操作：
新建数据库：
mysql> CREATE DATABASE mydb15_indexstu;
Query OK, 1 row affected (0.01 sec)
mysql> USE mydb15_indexstu;
Database changed
新建表：
mysql> CREATE TABLE student(
-> Sno INT PRIMARY KEY AUTO_INCREMENT,
-> Sname VARCHAR(30) NOT NULL UNIQUE,
-> Ssex VARCHAR(2) CHECK (Ssex='男' OR Ssex='女') NOT NULL,
-> Sage INT NOT NULL,
-> Sdept VARCHAR(10) DEFAULT '计算机'
-> );
mysql> CREATE TABLE Course(
-> Cno INT PRIMARY KEY NOT NULL,
-> Cname VARCHAR(20) NOT NULL
-> );
Query OK, 0 rows affected (0.02 sec)
mysql> create table SC(Sno int not null,Cno varchar(10) primary key not null,score int not null);
Query OK, 0 rows affected (0.02 sec)
表处理：
mysql> alter table student change Sage Sage smallint;
Query OK, 0 rows affected (0.07 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> create unique index course_index on course(cno);
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0
