Linear Interpolation in SQL - sql

Question

I work with crashes and mileage for the same year which is Year in table. Crashes are are there for every record, but annual mileage is not. NULLs for mileage could be at the beginning or at the end of the time period for certain customer. Also, couple of annual mileage records can be missing as well. I do not know how to overcome this. I try to do it in CASE statement but then I do not know how to code it properly. Issue needs to be resolved in SQL and use SQL Server.
This is how the output looks like and I need to have mileage for every single year for each customer.
The info I am pulling from is proprietary database and the records themselves should be untouched as is. I just need code in query which will modify my current output to output where I have mileage for every year. I appreciate any input!
Year
Customer
Crashes
Annual_Mileage
2009
123
5
3453453
2010
123
1
NULL
2011
123
0
54545
2012
123
14
376457435
2013
123
3
63453453
2014
123
4
NULL
2015
123
15
6346747
2016
123
0
NULL
2017
123
2
534534
2018
123
7
NULL
2019
123
11
NULL
2020
123
15
565435
2021
123
12
474567546
2022
123
7
NULL
Desired Results
Year
Customer
Crashes
Annual_Mileage
2009
123
5
3453453
2010
123
1
175399 (prior value is taken)
2011
123
0
54545
2012
123
14
376457435
2013
123
3
63453453
2014
123
4
34900100 (avg of 2 adjacent values)
2015
123
15
6346747
2016
123
0
3440641 (avg of 2 adjacent values)
2017
123
2
534534
2018
123
7
534534 ( prior value is taken)
2019
123
11
549985 (avg of 2 adjacent values)
2020
123
15
565435
2021
123
12
474567546
2022
123
7
474567546 (prior value is taken)
SELECT Year,
Customer,
Crashes,
CASE
WHEN Annual_Mlg IS NOT NULL THEN Annual_Mlg
WHEN Annual_Mlg IS NULL THEN
CASE
WHEN PREV.Annual_Mlg IS NOT NULL
AND NEXT.Annual_Mlg IS NOT NULL
THEN ( PREV.Annual_Mlg + NEXT.Annual_Mlg ) / 2
ELSE 0
END
END AS Annual_Mlg
FROM #table
The above code doesn't work, but I just need to start somehow and that what I have currently.
I understand what I need to do I just do not know how to code it in SQL.
After i applied row_number () function i got this output for first 2 clients and for the rest of the 4 clients row_number() function gave correct output. i have no idea why is that. I thought may be because i used "full join" before to combine mileage and crashes table?
enter image description here

Answer 1

Your use of #table tells me that you're using MS SQL Server (a temporary table, probably in a stored procedure).
You want to:
select all the rows in #table
joined with the matching row (if any) for the previous year, and
joined with the matching row (if any) for the next year
Then it's easy. Assuming the primary key on your #table is composed of the year and customer columns, something like this ought to do you:
select t.year ,
t.customer ,
t.crashes ,
annual_milage = coalesce(
t.annual_milage ,
( coalesce( p.annual_mileage, 0 ) +
coalesce( n.annual_mileage, 0 )
) / 2
)
from #table t -- take all the rows
left join #table p on p.year = t.year - 1 -- with the matching row for
and p.customer = t.customer -- the previous year (if any)
left join #table n on n.year = t.year + 1 -- and the matching row for
and n.customer = t.customer -- the next year (if any)
Notes:
What value you default to if the previous or next year doesn't exist is up to you (zero? some arbitrary value?)
Is the previous/next year guaranteed to be the current year +/- 1?
If not, you may have to use derived tables as the source for the
prev/next data, selecting the closest previous/next year (that sort
of thing rather complicates the query significantly).
Edited To Note:
If you have discontiguous years for each customer such that the "previous" and "next" years for a given customer are not necessarily the current year +/- 1, then something like this is probably the most straightforward way to find the previous/next year.
We use a derived table in our from clause, and assign a sequential number in lieu of year for each customer, using the ranking function row_number() function. This query, then
select row_nbr = row_number() over (
partition by x.customer
order by x.year
) ,
x.*
from #table x
would produce results along these lines:
row_nbr
customer
year
...
1
123
1992
...
2
123
1993
...
3
123
1995
...
4
123
2020
...
1
456
2001
...
2
456
2005
...
3
456
2020
...
And that leads us to this:
select year = t.year ,
customer = t.customer ,
crashes = t.crashes ,
annual_mileage = coalesce(
t.mileage,
coalesce(
t.annual_mileage,
(
coalesce(p.annual_mileage,0) +
coalesce(n.annual_mileage,0)
) / 2
),
)
from (
select row_nbr = row_number() over (
partition by x.customer
order by x.year
) ,
x.*
from #table x
) t
left join #table p on p.customer = t.customer and p.row_nbr = t.row_nbr-1
left join #table n on n.customer = t.customer and n.row_nbr = t.row_nbr+1