MySQL查询来统计同龄学生的频率？
您可以为此使用COUNT（*）和GROUPBY。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)