MySQL 中的 CAST 为 DECIMAL

我正在尝试在 MySQL 中转换为 Decimal，如下所示：

CAST((COUNT(*) * 1.5) AS DECIMAL(2))

我正在尝试将表中的行数（乘以 1.5）转换为该点后带有两位数字的浮点数。

SQL代码：

 SELECT CONCAT(Guardian.title, ' ', 
               Guardian.forename, ' ', 
               Guardian.surname) AS 'Guardian Name', 
               COUNT(*) AS 'Number of Activities', 
               (COUNT(*) * 1.5) AS 'Cost'
 FROM Schedule
 INNER JOIN Child ON Schedule.child_id = Child.id
 INNER JOIN Guardian ON Child.guardian = Guardian.id
 GROUP BY Guardian
 ORDER BY Guardian.surname, Guardian.forename ASC

它会产生一个错误：

#1064 - You have an error in your SQL syntax; check the manual that 
corresponds to your MySQL server version for the right syntax to use 
near 'CAST((COUNT(*) * 1.5) AS DECIMAL(12,2))' at line 1.

另一次尝试，这个演员阵容也不起作用：

 SELECT CONCAT(Guardian.title, ' ', 
               Guardian.forename, ' ', 
               Guardian.surname) AS 'Guardian Name', 
               COUNT(*) AS 'Number of Activities', 
               CAST((COUNT(*) * 1.5) AS DECIMAL(8,2)) AS 'Cost'
 FROM Schedule
 INNER JOIN Child ON Schedule.child_id = Child.id
 INNER JOIN Guardian ON Child.guardian = Guardian.id
 GROUP BY Guardian
 ORDER BY Guardian.surname, Guardian.forename ASC

如何使用mysql将整数转换为小数？

来自 MySQL 文档：定点类型（精确值）- DECIMAL、NUMERIC http://dev.mysql.com/doc/refman/5.5/en/fixed-point-types.html:

在标准 SQL 中，语法DECIMAL(M)相当于DECIMAL(M,0)

因此，您要转换为具有 2 位整数和 0 位小数的数字。试试这个：

CAST((COUNT(*) * 1.5) AS DECIMAL(12,2))