MySQL中的CAST到DECIMAL

Question

我试图像这样在MySQL中转换为Decimal:

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

我正在尝试将表中的行数(乘以1.5)转换为点后两位数的浮点数.

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.

另一个尝试,这个演员也不起作用:

 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从整数转换为十进制？

Answer 1

来自MySQL文档:定点类型(精确值) - DECIMAL,NUMERIC:

在标准SQL中,语法DECIMAL(M)等效于DECIMAL(M,0)

因此,您将转换为包含2个整数和0个十进制数字的数字.试试这个:

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

Answer 2

MySQL转换为十进制:

将裸整数转换为十进制:

select cast(9 as decimal(4,2));       //prints 9.00

将整数8/5转换为十进制:

select cast(8/5 as decimal(11,4));    //prints 1.6000

将字符串转换为小数:

select cast(".885" as decimal(11,3));   //prints 0.885

将两个int变量转换为十进制

mysql> select 5 into @myvar1;
Query OK, 1 row affected (0.00 sec)

mysql> select 8 into @myvar2;
Query OK, 1 row affected (0.00 sec)

mysql> select @myvar1/@myvar2;   //prints 0.6250

将十进制转换回字符串:

select cast(1.552 as char(10));   //shows "1.552"

Answer 3

DECIMAL有两部分：Precision和Scale。因此，您的查询的一部分将如下所示：

CAST((COUNT(*) * 1.5) AS DECIMAL(8,2))

Precision表示为值存储的有效位数。
Scale 表示小数点后可以存储的位数。