http://download.Oracle.com/docs/cd/E11882_01/server.112/e26088/functions088.htm#SQLRF00658
The LENGTH functionsreturn the length of char. LENGTH calculates length usingcharacters as defined by the input character set.
--返回以字符为单位的长度.
LENGTHB usesbytes instead of characters.
--返回以字节为单位的长度.
LENGTHC usesUnicode complete characters.
--返回以Unicode完全字符为单位的长度.
LENGTH2 usesUCS2 code points.
--返回以UCS2代码点为单位的长度.
LENGTH4 usesUCS4 code points.
--返回以UCS4代码点为单位的长度.
The exceptionsare LENGTHC, LENGTH2, and LENGTH4, which do not allow char tobe a CLOB or NCLOB. The return value is of data type NUMBER.If char has data type CHAR, then the length includes all trailingblanks. If char is null, then this function returns null.
The LENGTHB functionis supported for single-byte LOBs only. It cannot be used with CLOB and NCLOB datain a multibyte character set.
The followingexample uses the LENGTH function using a single-byte databasecharacter set:
SELECT LENGTH('CANDIDE') "Length incharacters" FROM DUAL;
Length in characters
--------------------
7
The next example assumes a double-bytedatabase character set.
SELECT LENGTHB ('CANDIDE') "Length inbytes" FROM DUAL;
Length in bytes
---------------
14
在不同的数据库，因为字符集的不同，LENGTHB得到的值可能会不一样。如ZHS16GBK采用两个byte位来定义一个汉字。而在UTF8，采用3个byte。
SYS@anqing1(rac1)> SELECT USERENV('LANGUAGE') FROM DUAL;
USERENV('LANGUAGE')
----------------------------------------------------
AMERICAN_AMERICA.ZHS16GBK
SQL>select length('安庆') from dual;
2
SQL>select lengthb('安庆') from dual;
4
SQL>select length('AnQing') from dual;
6
SQL>select lengthb('AnQing') from dual;
6
如果有中文，那么Length() != Lengthb()
如果没有中文，那么Length() == Lengthb()