你不能从一个在“编译”时不存在的表中SELECT，因为PL/SQL引擎会检查该表是否存在，如果不存在，就会引发ORA-00942: table or view does not exist。
如果你想从一个可能存在也可能不存在的表中执行SELECT，那么你需要通过将它 Package 在EXECUTE IMMEDIATE中来推迟执行该语句，直到运行时。
此外，您不需要先检查表是否存在;只要尝试查询表并捕获异常，如果它不存在：

DECLARE
  l_cnt INTEGER;
  table_does_not_exist EXCEPTION;
  PRAGMA EXCEPTION_INIT(table_does_not_exist, -942);
BEGIN
  EXECUTE IMMEDIATE 'select count(*) from Folder where id = 3' INTO l_cnt;

  IF (l_cnt > 0) THEN
    DBMS_OUTPUT.PUT_LINE('Value exists');
  ELSE
    DBMS_OUTPUT.PUT_LINE('Value does not exist');
  END IF;
EXCEPTION
  WHEN table_does_not_exist THEN
    DBMS_OUTPUT.PUT_LINE('Folder does not exist');
END;
/

fiddle

您可以使用 EXECUTE IMMEDIATE 对文件夹表运行查询：

declare
  own   varchar2(200);
  l_cnt integer;
begin
  select sys_context('userenv',
                    'current_schema')
    into own
    from dual;
  select count(*)
    into l_cnt
    from ALL_TABLES
   where Table_name = upper('Folder')
     and OWNER = own;
 if (l_cnt > 0) then
    execute immediate 'select count(*) from Folder where id = 3' into l_cnt;
    if (l_cnt > 0) then
      DBMS_OUTPUT.PUT_LINE('Value exists');
    else
      DBMS_OUTPUT.PUT_LINE('Value does not exist');
    end if;
else
  DBMS_OUTPUT.PUT_LINE('Folder does not exist');
end if;

结束;
请参阅有关EXECUTE IMMEDIATE的Oracle文档