VARCHAR2(size) [BYTE | CHAR]
Variable-length character string having maximum length size bytes or characters. Maximum size is 4000 bytes, and minimum is 1 byte or 1 character. You must specify size for VARCHAR2.
BYTE indicates that the column will have byte length semantics; CHAR indicates that the column will have character semantics.
varchar2(size, BYTE)
CHAR是字符,BYTE是字节。
http://www.oracle-base.com/articles/9i/character-semantics-and-globalization-9i.php