Recursive CTE with partition

Question

I have a table like this in MS SQL SERVER 2014:

ID|Race|Lap  
1 |21  |11  
2 |21  |NULL
3 |21  |NULL  
4 |21  |NULL  
5 |29  |65  
6 |29  |NULL  
7 |29  |NULL  
8 |29  |NULL 

I am trying to fill up the Lap column by adding 1 to it based on the first value. The partition is based on Race column. Something like this would be the end result:

ID|Race|Lap  
1 |21  |11  
2 |21  |12
3 |21  |13  
4 |21  |14  
5 |29  |65  
6 |29  |66  
7 |29  |67  
8 |29  |68  

There might be other ways of doing this but I would rather stick with recursive CTE. Is there any way to do this?

Answer 1

This would produce the expected result:

create table #demo (id int, race int, lap int)
insert into #demo values (1,21,11),(2,21,null),(3,21,null),(4,21,null),(5,29,65),(6,29,null),(7,29,null),(8,29,null);


with CTE as
(select race, ROW_NUMBER() over (partition by race  order by race) "extra_lap" from #demo where lap is null),
CTE2 as 
(select race, lap "lap" from #demo where lap is not null)
select race, lap from CTE2
union 
select CTE.race, CTE2.lap + CTE.extra_lap "lap" from CTE join CTE2 on CTE.race=CTE2.race

drop table #demo;

Answer 2

also another way using window functions :

create table #demo (id int, race int, lap int)
insert into #demo values (1,21,11),(2,21,null),(3,21,null),(4,21,null),(5,29,65),(6,29,null),(7,29,null),(8,29,null);

SELECT * , IIF(lap IS NULL , FIRST_VALUE(lap) OVER (PARTITION BY s.race ORDER BY id ) + RANK() OVER ( PARTITION BY s.race ORDER BY id ) -1 , lap)
 FROM #demo AS s

Answer 3

For this kind of cases Recursive CTE can be used as below with no need to partition.

--This temp table created to store sample values
SELECT Id,Race,Lap 
INTO #LapRecords 
FROM (VALUES (1,21,11),(2,21,NULL),(3,21,NULL),(4,21,NULL),(5,29,65),(6,29,NULL),(7,29,NULL),(8,29,NULL)) LapRecords (Id,Race,Lap);

--Recursive CTE
WITH LapRecordsCTE (Id,Race,Lap) AS
    (
        SELECT Id,Race,Lap FROM #LapRecords WHERE Lap IS NOT NULL
        UNION ALL 
        SELECT LapRecords.Id,LapRecords.Race,LapRecordsCTE.Lap+1
        FROM #LapRecords AS LapRecords JOIN LapRecordsCTE 
        ON LapRecords.Id=LapRecordsCTE.Id+1 AND LapRecords.Race=LapRecordsCTE.Race
    )

SELECT Id,Race,Lap FROM LapRecordsCTE ORDER BY Race,Lap;

--Temp table droped to clean up workspace
Drop Table #LapRecords