返回SQL数据类型，例如VARCHAR2.另外，我建议您不要使用WHEN OTHERS.另外，您不需要对double进行查询:

Return a SQL datatype, e.g. VARCHAR2. Also, I'd recommend against using WHEN OTHERS. Also, you don't need a query on dual:

create or replace 
function IS_NUMBER(str in varchar2) return varchar2
IS
  n number;
BEGIN
  n := to_number(str);
  return 'Y';
EXCEPTION WHEN VALUE_ERROR THEN
  return 'N';
END;