ТЕМА:

create trigger OSECTION_DELETE BEFORE DELETE on osection for each row

begin

    delete from oslist

    where oslist.os_id = :old.os_id;

    delete from osection

    where osection.os_parent = :old.os_id;

end;

ORA-04091 table string.string is mutating, trigger/function may not see it
Cause: A trigger (or a user defined PL/SQL function that is referenced in this
statement) attempted to look at (or modify) a table that was in the middle of
being modified by the statement which fired it.
Action: Rewrite the trigger (or function) so it does not read that table.

Oracle8i Application Developer's Guide - Fundamentals
Release 2 (8.1.6)
Part Number A76939-01

alter table osection add constaint osection_connstraint foreign key (os_parent) references osection (os_id) on delete cascade