SET @rank=0;	

update 
	core_objective_sub_items, 
	(select objective_ptr_id, @rank:=@rank+1 AS rank from core_task) as one
set 
	core_objective_sub_items.to_objective_id=one.objective_ptr_id
where 
	one.rank = core_objective_sub_items.id;

Auto increment in select statement

MySQL