LTRIM的原型是LTRIM(char [, set ])
LTRIM removes from the left end of char all of the characters contained in set. If you do not specify set, it defaults to a single blank. If char is a character literal, then you must enclose it in single quotes. Oracle Database begins scanning char from its first character and removes all characters that appear in set until reaching a character not in set and then returns the result.
LTRIM函数的第二个参数实际上是个集合，Oracle数据库会从char的第一个字符开始扫描，移除掉set集合中出现的字符，直到遇到一个不在set集合中的字符为止。
rtrim，trim 均如此。
