hello, can someone help me...

i have a table called ' book' with the following columns

book_hire_id
Book_Id
Customer_Id
Start_date
End_date
Book_Hired_Days

I have been trying to create a trigger so that no row within this table can be deleted if the start_date is equal to or less than the current date. I tried this:

CREATE or REPLACE TRIGGER triggerC
BEFORE DELETE on book_hire
for each row
BEGIN
update book_hire
where start_date > sysdate
end;
/

but this jus doesnt allow any deletion of rows from the table. any ideas?