在 MySQL 中计算已知不同值的出现次数
mysqlmysqli database
为此，您可以使用聚合函数 SUM()。让我们首先创建一个表 −
mysql> create table DemoTable636 ( StudentId int NOT NULL AUTO_INCREMENT PRIMARY KEY,StudentFirstName varchar(100) ); Query OK, 0 rows affected (0.51 sec)
使用 insert 命令在表中插入一些记录 −
mysql> insert into DemoTable636(StudentFirstName) values('John');
Query OK, 1 row affected (0.18 sec)
mysql> insert into DemoTable636(StudentFirstName) values('Robert');
Query OK, 1 row affected (0.13 sec)
mysql> insert into DemoTable636(StudentFirstName) values('Robert');
Query OK, 1 row affected (0.22 sec)
mysql> insert into DemoTable636(StudentFirstName) values('Sam');
Query OK, 1 row affected (0.20 sec)
mysql> insert into DemoTable636(StudentFirstName) values('Mike');
Query OK, 1 row affected (0.12 sec)
mysql> insert into DemoTable636(StudentFirstName) values('John');
Query OK, 1 row affected (0.14 sec)
mysql> insert into DemoTable636(StudentFirstName) values('Robert');
Query OK, 1 row affected (0.16 sec)
使用 select 语句显示表中的所有记录 −
mysql> select *from DemoTable636;
这将产生以下输出 −
+-----------+------------------+ | StudentId | StudentFirstName | +-----------+------------------+ | 1 | John | | 2 | Robert | | 3 | Robert | | 4 | Sam | | 5 | Mike | | 6 | John | | 7 | Robert | +-----------+------------------+ 7 rows in set (0.00 sec)
以下查询用于计算已知(或枚举)不同值的出现次数 −
mysql> select sum(StudentFirstName='John') AS JOHN_COUNT, sum(StudentFirstName='Robert') AS ROBERT_COUNT, sum(StudentFirstName='Sam') AS SAM_COUNT, sum(StudentFirstName='Mike') AS MIKE_COUNT from DemoTable636;
这将产生以下输出 −
+------------+--------------+-----------+------------+ | JOHN_COUNT | ROBERT_COUNT | SAM_COUNT | MIKE_COUNT | +------------+--------------+-----------+------------+ | 2 | 3 | 1 | 1 | +------------+--------------+-----------+------------+ 1 row in set (0.00 sec)
