trim, ltrim, rtrim

如果没有指定要截掉的字符，则默认为 空格；
trim一次只能截掉一种字符；
ltrime、rtrime一次可以截掉多种字符

1.trim

select trim(leading ',' from ',,,hello,,,')   ltrim,
       trim(trailing ',' from ',,,hello,,,')  rtrim,
       trim(both ',' from ',,,hello,,,')      btrim1,
       trim(',' from ',,,hello,,,')           btrim2
  from dual;

LTRIM      RTRIM      BTRIM1   BTRIM2
--------   --------   ------   ------
hello,,,   ,,,hello   hello    hello

2.ltrim, rtrim

select ltrim('<=,,,hello,,,=>', '<=,') ltrim,
       rtrim('<=,,,hello,,,=>', '>=,') rtrim
  from dual;

LTRIM        RTRIM    
----------   ----------
hello,,,=>   <=,,,hello