create table t (y date, value_pvt int, mmwl int)

insert into t 
select to_date('28/7/2018','dd/mm/yyyy')+level/24/60 
	   ,mod(level,3) 
	   ,null 
from dual 
connect by level<=5

with data 
  as (select y /* generate a sequence ordered by the y*/ 
			 ,value_pvt 
			 ,row_number() over(order by y) as rnk 
			 ,count(*) over(partition by 1) as tot_cnt 
	    from t 
	  ) 
    ,row_generator 
     as (select level as lvl /* generate a rows up and till 5*/ 
		   from dual 
		   connect by level<=(select tot_cnt from data where rownum=1) 
		 ) 
	, boundary_values 
     as (select a.lvl /* generate records for levels and specify the max and min values of the boundaries. Eg:for lvl=4 low_val=1 and hi_val=5*/ 
			   ,b.lvl as low_val 
			   ,a.lvl+b.lvl as hi_val 
	      from row_generator a 
          join row_generator b 
            on 1=1 
         where b.lvl + a.lvl <=(select tot_cnt from data where rownum=1)			 
	     )	 
    , computed_mmwl 
     as ( select lvl,max(diff) as diff 
           from (select m.lvl 
        		        ,n.value_pvt - o.value_pvt as diff 
                   from boundary_values m 
                   join data n 
                     on m.low_val=n.rnk 
                   join data o 
                     on m.hi_val=o.rnk    
                 ) group by lvl 
           union all 
           select max(rnk), 0  /* this union all adds the last record ie y('28/07/2018 00:05:00')|0   */ 
             from data 
        ) 
select to_char(a1.y,'dd/mm/yyyy hh24:mi:ss') as y 
       ,a1.value_pvt 
       ,b1.diff as new_mmwl 
 from data a1 
 join computed_mmwl b1 
   on a1.rnk=b1.lvl