将随机字符串值更新到oracle表中的现有记录

select  to_char(TO_DATE('2003/07/09', 'yyyy/mm/dd') + trunc(dbms_random.value(0,(TO_DATE('2003/07/09', 'yyyy/mm/dd')-TO_DATE('2018/07/09', 'yyyy/mm/dd')+1))),
                         'YYYY-MM') rDate   from dual

但出于某种原因,我的更新脚本正在为所有记录设置相同的值

CREATE TABLE tmp_del_me (    
    name varchar(255),
    year_month_started varchar(255)
);

insert into tmp_del_me (name)VALUES ('a');
insert into tmp_del_me (name)VALUES ('b');
insert into tmp_del_me (name)VALUES ('c');
insert into tmp_del_me (name)VALUES ('d');


UPDATE tmp_del_me
SET  year_month_started = (
                    select  to_char(TO_DATE('2003/07/09', 'yyyy/mm/dd') + trunc(dbms_random.value(0,(TO_DATE('2003/07/09', 'yyyy/mm/dd')-TO_DATE('2018/07/09', 'yyyy/mm/dd')+1))),
                     'YYYY-MM') rDate   from dual
                     )
            /

drop table tmp_del_me;