数据库是oracle，配置一个ddl触发器，针对direct用户，一旦有删表的操作，如果是匹配到我规定的20170901的表就记录下来
create table ddl_log(create_time date,op_table varchar2(128),op_user varchar2(30),op_type varchar2(64),is_send number default '0');--新建一张记录事件的表。
CREATE OR REPLACE TRIGGER DROP_DDL_TRIGGER AFTER DROP
ON DIRECT.SCHEMA
declare
col number;
begin
select count(*) into col from dba_objects WHERE owner='DIRECT' AND LAST_DDL_TIME=SYSDATE and OBJECT_NAME='test_20170901';
if col>0 then
insert into ddl_log(create_time,op_table,op_type) values((select sysdate from dual),'test_20170901','drop');
END IF;
END;
然后ddl触发器报错在select那一行说表或视图不存在，接着我有用另一种方法写触发器。
CREATE OR REPLACE TRIGGER DROP_DDL_TRIGGER AFTER DROP
ON DIRECT.SCHEMA
begin
if EXISTS(SELECT 1 FROM dba_objects WHERE owner='DIRECT' AND LAST_DDL_TIME=(SELECT SYSDATE FROM DUAL) AND OBJECT_NAME='test_20170901') THEN
insert into ddl_log(create_time,op_table,op_type) values(sysdate,'test_20170901','drop');
END IF;
END;
然后又报错说exists只能由于sql语句，请问怎么解决！求教
