mysql转换字符串为数字_mysql字符与数字转换
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.
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