นับจำนวนนักเรียนแยกตามอายุใน MySQL

ใช้ COUNT(*) ร่วม GROUP BY บนคอลัมน์อายุ

ตัวอย่าง

mysql> create table DemoTable (
    StudentId int NOT NULL AUTO_INCREMENT PRIMARY KEY,
    StudentAge int
);
Query OK, 0 rows affected (0.59 sec)

mysql> insert into DemoTable(StudentAge) values(16); Query OK, 1 row affected (0.15 sec)
mysql> insert into DemoTable(StudentAge) values(17); Query OK, 1 row affected (0.20 sec)
mysql> insert into DemoTable(StudentAge) values(18); Query OK, 1 row affected (0.13 sec)
mysql> insert into DemoTable(StudentAge) values(17); Query OK, 1 row affected (0.13 sec)
mysql> insert into DemoTable(StudentAge) values(17); Query OK, 1 row affected (0.23 sec)
mysql> insert into DemoTable(StudentAge) values(17); Query OK, 1 row affected (0.08 sec)
mysql> insert into DemoTable(StudentAge) values(18); Query OK, 1 row affected (0.17 sec)
mysql> insert into DemoTable(StudentAge) values(18); Query OK, 1 row affected (0.10 sec)
mysql> insert into DemoTable(StudentAge) values(19); Query OK, 1 row affected (0.12 sec)
mysql> insert into DemoTable(StudentAge) values(19); Query OK, 1 row affected (0.07 sec)
mysql> insert into DemoTable(StudentAge) values(16); Query OK, 1 row affected (0.13 sec)
mysql> insert into DemoTable(StudentAge) values(16); Query OK, 1 row affected (0.14 sec)
mysql> insert into DemoTable(StudentAge) values(15); Query OK, 1 row affected (0.16 sec)

นับความถี่

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

+------------+--------------+
| StudentAge | AgeFrequency |
+------------+--------------+
| 16         | 3            |
| 17         | 4            |
| 18         | 3            |
| 19         | 2            |
| 15         | 1            |
+------------+--------------+