I am working with an Oracle 10g Database.
I have the following two tables:
T_DEBTOR : - ID_DEBTOR - HEADER T_ELEMENT : -
Have you tried
update T_ELEMENT elt set elt.INSURER = ( select HEADER from T_DEBTOR debtor where debtor.HEADER is not null and debtor.ID_DEBITEUR = elt.ID_DEBITEUR) where not elt.ID_DEBITEUR is null;