新建数据库
mysql> create database mydb15_indexstu;
Query OK, 1 row affected (0.03 sec)mysql> use mydb15_indexstu;
Database changed新建表
创建学生信息表
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' not null );
Query OK, 0 rows affected (0.08 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)创建课程表
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)创建选课表
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)处理表格
1. 修改 Student 表中的 Sage 字段数据类型
mysql> ALTER TABLE Student MODIFY COLUMN Sage SMALLINT;
Query OK, 0 rows affected (0.13 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 表中的 Cno 课程号字段设置索引，并查看索引
mysql> CREATE INDEX idx_Cno ON Course(Cno);
Query OK, 0 rows affected (0.05 sec)
Records: 0 Duplicates: 0 Warnings: 0mysql> SHOW INDEXES FROM Course;
+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| course | 0 | PRIMARY | 1 | Cno | A | 0 | NULL | NULL | | BTREE | | | YES | NULL |
| course | 1 | idx_Cno | 1 | Cno | A | 0 | NULL | NULL | | BTREE | | | YES | NULL |
+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
2 rows in set (0.03 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> ALTER TABLE SC ADD CONSTRAINT SC_INDEX PRIMARY KEY (Sno, Cno);
Query OK, 0 rows affected (0.05 sec)
Records: 0 Duplicates: 0 Warnings: 0mysql> SHOW INDEXES FROM SC;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| sc | 0 | PRIMARY | 1 | Sno | A | 0 | NULL | NULL | | BTREE | | | YES | NULL |
| sc | 0 | PRIMARY | 2 | Cno | A | 0 | NULL | NULL | | BTREE | | | YES | NULL |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
2 rows in set (0.03 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 Student.Sname, Student.Ssex, Course.Cname, SC.Score-> FROM Student-> JOIN SC ON Student.Sno = SC.Sno-> JOIN Course ON SC.Cno = Course.Cno;
Query OK, 0 rows affected (0.04 sec)mysql> desc stu_info;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| Sname | varchar(30) | NO | | NULL | |
| Ssex | varchar(2) | NO | | NULL | |
| Cname | varchar(20) | NO | | NULL | |
| Score | int | NO | | NULL | |
+-------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)5. 删除所有索引
mysql> DROP INDEX idx_Cno ON Course;
Query OK, 0 rows affected (0.04 sec)
Records: 0 Duplicates: 0 Warnings: 0mysql> DROP INDEX SC_INDEX ON SC;
Query OK, 0 rows affected (0.04 sec)
Records: 0 Duplicates: 0 Warnings: 0总结
通过执行这些 SQL 语句，可以完成以下任务：
- 修改字段数据类型：如将
Sage字段的数据类型修改为SMALLINT，以优化存储。 - 创建索引：通过为字段（如
Cno）创建索引，优化查询性能。 - 创建主键索引：通过为多个字段（如
Sno和Cno）创建组合主键，保证数据的唯一性，并加速查询。 - 创建视图：通过视图简化查询，方便查询多个表的数据，并展示学生的姓名、性别、课程和成绩。
- 删除索引：如果索引不再需要，可以删除，以节省空间和提高性能。
确保在执行这些操作时，仔细检查数据库表结构和数据的完整性，尤其是在删除索引和修改字段类型时。
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