MySQL查询来统计同龄学生的频率？

您可以为此使用COUNT（*）和GROUP BY。让我们首先创建一个表-

mysql> create table DemoTable
   (
   StudentId int NOT NULL AUTO_INCREMENT PRIMARY KEY,
   StudentAge int
   );

使用插入命令在表中插入记录-

mysql> insert into DemoTable(StudentAge) values(16);
mysql> insert into DemoTable(StudentAge) values(17);
mysql> insert into DemoTable(StudentAge) values(18);
mysql> insert into DemoTable(StudentAge) values(17);
mysql> insert into DemoTable(StudentAge) values(17);
mysql> insert into DemoTable(StudentAge) values(17);
mysql> insert into DemoTable(StudentAge) values(18);
mysql> insert into DemoTable(StudentAge) values(18);
mysql> insert into DemoTable(StudentAge) values(19);
mysql> insert into DemoTable(StudentAge) values(19);
mysql> insert into DemoTable(StudentAge) values(16);
mysql> insert into DemoTable(StudentAge) values(16);
mysql> insert into DemoTable(StudentAge) values(15);

使用select语句显示表中的所有记录-

mysql> select * from DemoTable;

这将产生以下输出-

+-----------+------------+
| StudentId | StudentAge |
+-----------+------------+
| 1         | 16         |
| 2         | 17         |
| 3         | 18         |
| 4         | 17         |
| 5         | 17         |
| 6         | 17         |
| 7         | 18         |
| 8         | 18         |
| 9         | 19         |
| 10        | 19         |
| 11        | 16         |
| 12        | 16         |
| 13        | 15         |
+-----------+------------+
13 rows in set (0.00 sec)

以下是计算频率的查询-

mysql> select StudentAge,count(*) AS `AgeFrequency` from DemoTable group by StudentAge;

这将产生以下输出-

+------------+--------------+
| StudentAge | AgeFrequency |
+------------+--------------+
| 16         | 3            |
| 17         | 4            |
| 18         | 3            |
| 19         | 2            |
| 15         | 1            |
+------------+--------------+
5 rows in set (0.05 sec)