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：

FM

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.

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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）