Здрасти, значи хрумна ми този вариант (използвам T-SQL диалекта понеже ти не уточни базата)

insert into #trees (parent , title)
select title, parent from trees;

delete from #trees where title is null;

update #trees set title = parent, parent = null
where not exists (select title from #trees t1 where t1.title = #trees.parent )

delete from trees;

insert into trees select * from #trees;