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