根据题目要求，完成下列任务：

 新建表和数据库

//建立数据库
mysql> create database mydb15_indexstu;
Query OK, 1 row affected (0.00 sec)//使用数据库
mysql> use mydb15_indexstu;
Database changed//创建表student
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' not null);
Query OK, 0 rows affected (0.02 sec)mysql> desc student;
+-------+-------------+------+-----+-----------+----------------+
| Field | Type        | Null | Key | Default   | Extra          |
+-------+-------------+------+-----+-----------+----------------+
| sno   | int         | NO   | PRI | NULL      | auto_increment |
| sname | varchar(30) | NO   | UNI | NULL      |                |
| ssex  | varchar(2)  | NO   |     | NULL      |                |
| sage  | int         | NO   |     | NULL      |                |
| sdept | varchar(10) | NO   |     | 计算机    |                |
+-------+-------------+------+-----+-----------+----------------+
5 rows in set (0.00 sec)//创建表course
mysql> create table course (cno int primary key not null,cname varchar(20) not null);
Query OK, 0 rows affected (0.02 sec)mysql> desc course;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| cno   | int         | NO   | PRI | NULL    |       |
| cname | varchar(20) | NO   |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)//创建表sc
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)mysql> desc sc;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| sno   | int         | NO   |     | NULL    |       |
| cno   | varchar(10) | NO   | PRI | NULL    |       |
| score | int         | NO   |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

1.修改student 表中年龄(sage)字段属性，数据类型由int 改变为smallint

mysql> alter table student modify sage smallint;
Query OK, 0 rows affected (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 0mysql> desc student;
+-------+-------------+------+-----+-----------+----------------+
| Field | Type        | Null | Key | Default   | Extra          |
+-------+-------------+------+-----+-----------+----------------+
| sno   | int         | NO   | PRI | NULL      | auto_increment |
| sname | varchar(30) | NO   | UNI | NULL      |                |
| ssex  | varchar(2)  | NO   |     | NULL      |                |
| sage  | smallint    | YES  |     | NULL      |                |
| sdept | varchar(10) | NO   |     | 计算机    |                |
+-------+-------------+------+-----+-----------+----------------+
5 rows in set (0.00 sec)

 2.为Course表中Cno 课程号字段设置索引,并查看索引

mysql> create index index_cno on course(cno);
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0mysql> show create table course\G
*************************** 1. row ***************************Table: course
Create Table: CREATE TABLE `course` (`cno` int NOT NULL,`cname` varchar(20) NOT NULL,PRIMARY KEY (`cno`),KEY `index_cno` (`cno`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
1 row in set (0.00 sec)

3.为SC表建立按学号(sno)和课程号(cno)组合的升序的主键索引，索引名为SC_INDEX

mysql> create index sc_index on sc(sno,cno asc);
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0mysql> desc sc;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| sno   | int         | NO   | MUL | NULL    |       |
| cno   | varchar(10) | NO   | PRI | NULL    |       |
| score | int         | NO   |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

 4.创建一视图 stu info,查询全体学生的姓名，性别，课程名，成绩

mysql> create view stu_info as-> select sname,ssex,cname,score-> from student,sc,course;
Query OK, 0 rows affected (0.00 sec)mysql> select * from stu_info;
Empty set (0.00 sec)mysql> show tables;
+---------------------------+
| Tables_in_mydb15_indexstu |
+---------------------------+
| course                    |
| sc                        |
| stu_info                  |
| student                   |
+---------------------------+
4 rows in set (0.00 sec)

 5.删除所有索引

mysql> drop index sc_index on sc;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0mysql> drop index index_cno on course;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0mysql> show create table sc\G
*************************** 1. row ***************************Table: sc
Create Table: CREATE TABLE `sc` (`sno` int NOT NULL,`cno` varchar(10) NOT NULL,`score` int NOT NULL,PRIMARY KEY (`cno`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
1 row in set (0.00 sec)mysql> show create table course\G
*************************** 1. row ***************************Table: course
Create Table: CREATE TABLE `course` (`cno` int NOT NULL,`cname` varchar(20) NOT NULL,PRIMARY KEY (`cno`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
1 row in set (0.00 sec)