Question

RecordNo    Speed  
--------    -----
1           0
2           0
3           0
4           0
5           23
6           66
7           48
8           0
9           31
10          0
11          34
12          23

The above data shows the speed of vehicle over a time period, given the above data I need to achieve the result below:

RecordNo    Speed   LastAcceleration
--------    -----   ----------------
1           0       0
2           0       0
3           0       0
4           0       0
5           23      23
6           66      66
7           48      66
8           0       66
9           31      31
10          0       31
11          34      34
12          23      34

The code below is almost there but falls over on Recordno 8:

select
curr.recordno,curr.speed 
,CASE WHEN curr.speed >= ISNULL(prev.speed,0) THEN curr.speed
    ELSE (
            SELECT MAX(speed) FROM speedtest 
            WHERE recordno between (CASE WHEN curr.speed >= prev.speed then curr.recordindex else prev.recordno end ) and curr.recordno 
          ) 
END as LastAcceleration
From speedtest prev RIGHT JOIN speedtest curr
on prev.vrm = curr.vrm
and prev.recordno+1 = curr.recordno
order by curr.recordno

I think I’ve been staring at this one too long. I’ve tried self-joins with correlated sub-queries but think I’m missing something obvious? This is for a 2008 project so doesn’t have to work with any versions prior to that. Any help would be appreciated.

Answer 1

Here you go! You basically missed the part where you need to check for the latest acceleration whenever its not accelerating (the not exists portion of my query moreorless)

create table #t (recordno int, speed int)
insert into #t
select 1,           0
union select 2,           0
union select 3,           0
union select 4,           0
union select 5,           23
union select 6,           66
union select 7,           48
union select 8,           0
union select 9,           31
union select 10,          0
union select 11,          34
union select 12,          23

;with acc as (
select
    t1.*,
    case when t1.speed >= isnull(t2.speed, 0) then t1.speed else -1 end as 'accel'
from
    #t t1
    left join #t t2
        on  t1.recordno = t2.recordno + 1
)

select 
    a.recordno, a.speed, 
    case 
        when a.accel >= 0 then a.accel 
        else (
            select accel 
            from acc a1 
            where a1.recordno < a.recordno and a1.accel >= 0 
                and not exists (
                    select 1 
                    from acc a2 
                    where a2.recordno < a.recordno and a2.accel >= 0 and a2.recordno > a1.recordno)
            )
    end
from
    acc a

drop table #t