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本节内容：
mysql字符与数字转换的方法
1，将字符的数字转成数字，比如’0’转成0可以直接用加法实现。
例如：将pony表中的d 进行排序，可d的定义为varchar：
复制代码 代码示例:
select * from pony order by (d+0)
2，在进行ifnull处理时，比如 ifnull(a/b,’0′) 会导致 a/b成了字符串，因此需要把’0’改成0。
3，比较数字和varchar时，比如a=11，b=”11ddddd”;
则 select 11=”11ddddd”相等
绝对比较可以这样：
复制代码 代码示例:
select binary 11 =binary “11ddddd”
附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：
1，可用的类型
2，二进制,同带binary前缀的效果 : BINARY
3，字符型,可带参数 : CHAR()
4，日期 : DATE
5，时间: TIME
6，日期时间型 : DATETIME
7，浮点数 : DECIMAL
8，整数 : SIGNED
9，无符号整数 : UNSIGNED
附2，
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:
复制代码 代码示例:
mysql> SELECT 1+’1′;
-> 2
If you use a number in string context, the number automatically is converted to a BINARY string.
复制代码 代码示例:
mysql> SELECT CONCAT(‘hello you ‘,2);
-> ‘hello you 2’
MySQL supports arithmetic with both signed and unsigned 64-bit values. If you are using numeric operators (such as + or -) and one of the operands is an unsigned integer, the result is unsigned. You can override this by using the SIGNED and UNSIGNED cast operators to cast the operation to a signed or unsigned 64-bit integer, respectively.
复制代码 代码示例:
mysql> SELECT CAST(1-2 AS UNSIGNED)
-> 18446744073709551615
mysql> SELECT CAST(CAST(1-2 AS UNSIGNED) AS SIGNED);
-> -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, DECIMAL column values are regarded as floating-point values.)
复制代码 代码示例:
mysql> SELECT CAST(1 AS UNSIGNED) – 2.0;
-> -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, CONVERT() and CAST() return NULL when the NO_ZERO_DATE SQL mode is enabled. As of MySQL 5.0.4, they also produce a warning.
有关MYSQL字符与数字转换的方法，就介绍这些吧，希望对大家有所帮助。
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