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?

• mustaccio: How would a recursive CTE help here? Sounds like an X-Y problem to me.
• SQLserving: @mustaccio well the X seems to be too complicated but here is a simpler version I asked which did not get any answer so this is a further simplification.

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;

• SQLserving: Thanks for the help that worked!

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

• SQLserving: that works and is much simpler! I hadnt known about FIRST_VALUE, thanks for that!

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

• eshirvana: that has a flaw , if you remove a row in the middle of group , It will fail to returns the rest ( for example id =7 or id =2 ) , you can't trust id