date not initialized

I have a table like this :
table boocks (name varchar , return_date date)

The field return_date is not initialized :
insert into books values (name1);
insert into books values (namex);

The field return_date is updated when the books is returned like this :

update books set return_date='10-06-1999' where name=xxx;

How can i select all the item where the return_date is not updated ?

a request like this :

select * from books where return_date='' ????

(actually i initialised the return_date with the values '01-01-0001')

thank you.

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