(1) 在students表中，查询年龄大于25岁，且为男性的同学的名字和年龄

MariaDB [hellodb]> SELECT * FROM students WHERE Age>'25' and Gender='M';
+-------+--------------+-----+--------+---------+-----------+
| StuID | Name         | Age | Gender | ClassID | TeacherID |
+-------+--------------+-----+--------+---------+-----------+
|     3 | Xie Yanke    |  53 | M      |       2 |        16 |
|     4 | Ding Dian    |  32 | M      |       4 |         4 |
|     5 | Yu Yutong    |  26 | M      |       3 |         1 |
|     6 | Shi Qing     |  46 | M      |       5 |      NULL |
|    13 | Tian Boguang |  33 | M      |       2 |      NULL |
|    24 | Xu Xian      |  27 | M      |    NULL |      NULL |
|    25 | Sun Dasheng  | 100 | M      |    NULL |      NULL |
+-------+--------------+-----+--------+---------+-----------+
7 rows in set (0.00 sec)

(2) 以ClassID为分组依据，显示每组的平均年龄

MariaDB [hellodb]> select classid,avg(age) from students where classid is not null group by classid order by classid;
+---------+----------+
| 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 (0.00 sec)

(3) 显示第2题中平均年龄大于30的分组及平均年龄

MariaDB [hellodb]> select classid,avg(age) from students where age > 30 and classid is not null group by classid order by classid;
+---------+----------+
| classid | avg(age) |
+---------+----------+
|       2 |  43.0000 |
|       4 |  32.0000 |
|       5 |  46.0000 |
+---------+----------+
3 rows in set (0.00 sec)

(4) 显示以L开头的名字的同学的信息

MariaDB [hellodb]> select * from students where Name rlike 'L.*';
+-------+--------------+-----+--------+---------+-----------+
| StuID | Name         | Age | Gender | ClassID | TeacherID |
+-------+--------------+-----+--------+---------+-----------+
|     8 | Lin Daiyu    |  17 | F      |       7 |      NULL |
|    10 | Yue Lingshan |  19 | F      |       3 |      NULL |
|    14 | Lu Wushuang  |  17 | F      |       3 |      NULL |
|    17 | Lin Chong    |  25 | M      |       4 |      NULL |
+-------+--------------+-----+--------+---------+-----------+
4 rows in set (0.00 sec)

数据库授权magedu用户，允许192.168.1.0/24网段可以连接mysql

MariaDB [(none)]> GRANT ALL ON *.* TO magedu@'192.168.1.%' IDENTIFIED BY 'magedu';
Query OK, 0 rows affected (0.00 sec)

MariaDB [(none)]> select USER,HOST from mysql.user;
+--------+-----------------------+
| USER   | HOST                  |
+--------+-----------------------+
| root   | 127.0.0.1             |
| magedu | 192.168.1.%           |
| root   | ::1                   |
|        | localhost             |
| root   | localhost             |
|        | localhost.localdomain |
| root   | localhost.localdomain |
+--------+-----------------------+
8 rows in set (0.00 sec)