TO_SINGLE_BYTE returns char with all its multibyte characters converted to their corresponding single-byte characters. Char can be of data type CHAR, VARCHAR2, NCHAR, or NVARCHAR2. The value returned is in the same data type as char.
Any multibyte characters in char that have no single-byte equivalents appear in the output as multibyte characters. This function applies if your database character set contains single-byte and multibyte characters.