1、如何将 hellodb_innodb.sql导入到数据库中
答：
方法1：在命令行中
[root@centos8 ~]#mysql < hellodb_innodb.sql
方法2：在数据库中
MariaDB [(none)]> source hellodb_innodb.sql
2、在学生表中，查询年龄大于25岁，且为男性的同学的名字和年龄
答：
MariaDB [hellodb]> SELECT name,age FROM students WHERE age > 25 AND gender='M';
+--------------+-----+
| name | age |
+--------------+-----+
| Xie Yanke | 53 |
| Ding Dian | 32 |
| Yu Yutong | 26 |
| Shi Qing | 46 |
| Tian Boguang | 33 |
| Xu Xian | 27 |
| Sun Dasheng | 100 |
+--------------+-----+
7 rows in set (0.000 sec)
3、在学生表中，以ClassID为分组依据，查询显示每组的平均年龄
答：
MariaDB [hellodb]> SELECT classid,avg(age) FROM students GROUP BY classid HAVING classid IS NOT NULL;
+---------+----------+
| classid | avg(age) |
+---------+----------+
| 1 | 20.5000 |
| 2 | 36.0000 |
| 3 | 20.2500 |
| 4 | 24.7500 |
| 5 | 46.0000 |
| 6 | 20.7500 |
| 7 | 19.6667 |
+---------+----------+
7 rows in set (0.000 sec)
4、显示第3题中平均年龄大于30的分组及平均年龄
答：
MariaDB [hellodb]> SELECT classid,avg(age) FROM students GROUP BY classid HAVING classid IS NOT NULL AND avg(age) > 30;
+---------+----------+
| classid | avg(age) |
+---------+----------+
| 2 | 36.0000 |
| 5 | 46.0000 |
+---------+----------+
2 rows in set (0.001 sec)
5、显示以L开头的名字的同学的信息
答：
MariaDB [hellodb]> SELECT * FROM students WHERE name LIKE 'L%';
+-------+-------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+-------------+-----+--------+---------+-----------+
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
+-------+-------------+-----+--------+---------+-----------+
3 rows in set (0.000 sec)
6、显示老师ID非空的同学的相关信息
答：
MariaDB [hellodb]> SELECT * FROM students WHERE teacherid IS NOT NULL;
+-------+-------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+-------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
+-------+-------------+-----+--------+---------+-----------+
5 rows in set (0.000 sec)
7、students表中，查询以年龄排序后的数据，并且显示年龄最大的前10位同学的信息
答：
MariaDB [hellodb]> SELECT * FROM students ORDER BY age DESC LIMIT 10;
+-------+--------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+--------------+-----+--------+---------+-----------+
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
+-------+--------------+-----+--------+---------+-----------+
10 rows in set (0.000 sec)
8、students表中，查询年龄大于等于20岁，小于等于25岁的同学的信息
答：
MariaDB [hellodb]> SELECT * FROM students WHERE age BETWEEN 20 AND 25;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
+-------+---------------+-----+--------+---------+-----------+
10 rows in set (0.000 sec)
9、以ClassID分组，显示每班的同学的人数
答：
MariaDB [hellodb]> SELECT classid,count(name) '每班人数' FROM students GROUP BY classid HAVING classid IS NOT NULL;
+---------+--------------+
| classid | 每班人数 |
+---------+--------------+
| 1 | 4 |
| 2 | 3 |
| 3 | 4 |
| 4 | 4 |
| 5 | 1 |
| 6 | 4 |
| 7 | 3 |
+---------+--------------+
7 rows in set (0.000 sec)
10、以ClassID分组，显示其平均年龄大于25的班级
答：
MariaDB [hellodb]> SELECT classid,avg(age) '平均年龄' FROM students WHERE classid IS NOT NULL GROUP BY classid HAVING avg(age)>25;
+---------+--------------+
| classid | 平均年龄 |
+---------+--------------+
| 2 | 36.0000 |
| 5 | 46.0000 |
+---------+--------------+
2 rows in set (0.000 sec)