--给个例子
declare
num number;
begin
select count(1) into num from all_tables where table_name='TESTA';
if num>0 then
execute immediate 'drop table TESTA';
end if;
execute immediate 'create table TESTA as select * from dept';
end;
/

------解决方案--------------------
SQL code

SET serveroutput ON;
declare 
  i integer;
BEGIN
  i:=0;
  SELECT count(*) INTO i FROM user_tables WHERE table_name='TB';
  IF i=1 THEN
    EXECUTE IMMEDIATE 'drop table tb';
    dbms_output.put_line('删除成功');
  ELSE
    dbms_output.put_line('删除失败');
  END IF;
end;