Mysql点滴－MYSQL字符数字转换

 

1.

将字符的数字转成数字，比如'0'转成0可以直接用加法来实现

例如：将pony表中的d 进行排序，可d的定义为varchar，可以这样解决

select * from pony order by (d+0)

2.

在进行ifnull处理时，比如 ifnull(a/b,'0') 这样就会导致 a/b成了字符串，因此需要把'0'改成0，即可解决此困扰

3比较数字和varchar时，比如a=11，b="11ddddd";

则 select 11="11ddddd"相等

若绝对比较可以这样：

select binary 11 =binary "11ddddd"

附录1

字符集转换 : CONVERT(xxx USING gb2312)

类型转换和SQL Server一样,就是类型参数有点点不同 : CAST(xxx AS 类型) , CONVERT(xxx,类型)，类型必须用下列的类型：

可用的类型　
二进制,同带binary前缀的效果 : BINARY
字符型,可带参数 : CHAR()
日期 : DATE
时间: TIME
日期时间型 : DATETIME
浮点数 : DECIMAL
整数 : SIGNED
无符号整数 : UNSIGNED

附录2

http://dev.mysql.com/doc/refman/5.0/en/cast-functions.htmlmysql转换函数之cast

To cast a string to a numeric value in numeric context, you normally do not have to do anything other than to use the string value as though it were a number:

mysql> DE>SELECT 1+'1';DE>

-> 2

If you use a number in string context, the number automatically is converted to aDE>BINARYDE> string.

mysql> DE>SELECT CONCAT('hello you ',2);DE>

-> 'hello you 2'

MySQL supports arithmetic with both signed and unsigned 64-bit values. If you are using numeric operators (such asDE>+DE> or DE>-DE>) and one of the operands is an unsigned integer, the result is unsigned. You can override this by using theDE>SIGNEDDE> and DE>UNSIGNEDDE> cast operators to cast the operation to a signed or unsigned 64-bit integer, respectively.

mysql> DE>SELECT CAST(1-2 AS UNSIGNED)DE>

-> 18446744073709551615

mysql> DE>SELECT CAST(CAST(1-2 AS UNSIGNED) AS SIGNED);DE>

-> -1

Note that if either operand is a floating-point value, the result is a floating-point value and is not affected by the preceding rule. (In this context,DE>DECIMALDE> column values are regarded as floating-point values.)

mysql> DE>SELECT CAST(1 AS UNSIGNED) - 2.0;DE>

-> -1.0

If you are using a string in an arithmetic operation, this is converted to a floating-point number.

If you convert a “zero” date string to a date, DE>CONVERT()DE> and DE>CAST()DE> return DE>NULLDE> when the DE>NO_ZERO_DATEDE> SQL mode is enabled. As of MySQL 5.0.4, they also produce a warning.