问问oracle触发器

create table AAA
(
A VARCHAR2(44),
B VARCHAR2(44),
C VARCHAR2(44),
D VARCHAR2(44),
E VARCHAR2(44)
)
CREATE OR REPLACE TRIGGER "UPDATE_AAA_CDD" BEFORE
INSERT
OR UPDATE OF "A", "B", "C", "D", "E" ON "DZSW"."AAA" REFERENCING OLD AS OLD_VALUE NEW AS AAA FOR EACH ROW
when (aaa.c='2' )
begin

:aaa.d:='f';

end;
我怎么让aaa.c='2'时aaa.d的值等于aaa.a的值呢？