在MySQL中转换为十进制

Question

我尝试在MySQL中转换为小数，如下所示：

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

我尝试将表中的行数(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.

再试一次，这个造型也不起作用：

 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？

Answer 1

来自MySQL文档：Fixed-Point Types (Exact Value) - DECIMAL, NUMERIC

在标准SQL中，语法DECIMAL(M)等同于DECIMAL(M,0)

因此，您将转换为具有2个整数位和0个小数位的数字。试着这样做：

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

Answer 2

MySQL转换为小数：

将纯整数转换为小数：

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

将8/5的整数转换为小数：

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

将字符串转换为十进制：

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

将两个整型变量转换为十进制

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有两个部分：Precision和Scale。因此，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表示小数点后可存储的位数。