Oracle中to_char函数处理数字时前面的空格问题

自打我用to_char来转换数字就发现了这个问题，即转换结果字符串最前面多了一个空格。我一直用trim再处理一下，没有关注过原因。今天上网查了一下，看到了一个说法及解决方案。

Oracle to_char():数字转字符串，结果添加空格：

select to_char(12,'00') from dual

字符串是 ' 12',前面多了一个空格，

select length(to_char(12,'00')) from dual

返回结果是:3

返回的字符串总是前面有个空格。原因：

那个空格位置是放符号的，正的数字就空了，负的就是一个‘-’号而没有空格。

这是网上流传的原因：

Fill mode. Oracle uses blank characters to fill format elements to a constant width equal to the largest element for the relevant format model in the current session language. For example, when NLS_LANGUAGE is AMERICAN, the largest element for MONTH is SEPTEMBER, so all values of the MONTH format element are padded to 9 display characters. This modifier suppresses blank padding in the return value of the TO_CHAR function:

In a datetime format element of a TO_CHAR function, this modifier suppresses blanks in subsequent character elements (such as MONTH) and suppresses leading zeroes for subsequent number elements (such as MI) in a date format model. Without FM, the result of a character element is always right padded with blanks to a fixed length, and leading zeroes are always returned for a number element. With FM, which suppresses blank padding, the length of the return value may vary.

In a number format element of a TO_CHAR function, this modifier suppresses blanks added to the left of the number, so that the result is left-justified in the output buffer. Without FM, the result is always right-justified in the buffer, resulting in blank-padding to the left of the number.

Track back

解决办法：

select   trim(to_char(12,'00'))   from   dual 
或者 
select   to_char(12,'fm00')   from   dual 

总结： 
select   to_char(12,'00000')   from   dual 结果为： 
00012（1个空格） 
select   to_char(12,'99999')   from   dual 结果为： 
12（4个空格） 
select   to_char(-12,'00000')   from   dual 结果为： 
-00012（无个空格） 
select   to_char(-12,'99999')   from   dual 结果为： 
   -12（3个空格）