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)
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.html mysql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 a DE>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 as DE>+DE> or DE>-DE>) and one of the operands is an unsigned integer, the result is unsigned. You can override this by using the DE>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.