create table Demo (id int, Time datetime, value numeric(3,1))
go

insert into demo (id, time, value)
select 1, '10:00:00', 4 
union select 2, '10:05:00', 6 
union select 3, '10:10:00', 3 
union select 4, '10:15:00', 7 
union select 5, '10:20:00', 8 
union select 6, '10:25:00', 2 
union select 7, '10:30:00', 4 
go


select a.id, a.time, a.value, (a.value + isnull(b.value, a.value))/2 as AvgCurrentAndPref
from demo as a left join demo as B on a.id = b.id+1
  
go
drop table demo