Running "Group By" Ordinal Counter Based on a "Flip" Column - sql-server

Question

Usually I'm decent at set-based tsql problems. But this one is beating me.
I've been working 3 days on converting a while-loop procedure into a setbased one. I've gotten to the point below.......but can't make the final jump.
I have the following rows. MyOrdinal will be "in order" ... and a second column (MyMarker) will alternate between having a value and being null. Whenever this "flip" occurs on MyMarker, I would like to increment a "group by" ordinal counter by one. Whenever the "flip" values are non-null or null, these are grouped together as a set.
I've tried several things, but it was too ugly to post. That and since moving to ORM, I don't spend as much time in the tsql anymore.
declare #Holder table ( MyOrdinal int not null , MyMarker int , MyGroupNumber int )
INSERT INTO #Holder (MyOrdinal, MyMarker)
Select 1 , 1
union all Select 2, 2
union all Select 3, null
union all Select 4, 3
union all Select 5, 4
union all Select 6, 5
union all Select 7, 6
union all Select 8, 7
union all Select 9, 8
union all Select 10, 9
union all Select 11, 10
union all Select 12, 11
union all Select 13, 12
union all Select 14, 13
union all Select 15, 14
union all Select 16, 15
union all Select 17, null
union all Select 18, null
union all Select 19, null
union all Select 20, 16
union all Select 21, 17
union all Select 22, 18
union all Select 23, null
union all Select 24, null
union all Select 25, 19
union all Select 26, 20
union all Select 27, null
union all Select 28, 21
Select * from #Holder
Desired Output
| MyOrdinal | MyMarker | MyGroupNumber |
|-----------|----------|---------------|
| 1 | 1 | 1 |
| 2 | 2 | 1 |
| 3 | null | 2 |
| 4 | 3 | 3 |
| 5 | 4 | 3 |
| 6 | 5 | 3 |
| 7 | 6 | 3 |
| 8 | 7 | 3 |
| 9 | 8 | 3 |
| 10 | 9 | 3 |
| 11 | 10 | 3 |
| 12 | 11 | 3 |
| 13 | 12 | 3 |
| 14 | 13 | 3 |
| 15 | 14 | 3 |
| 16 | 15 | 3 |
| 17 | null | 4 |
| 18 | null | 4 |
| 19 | null | 4 |
| 20 | 16 | 5 |
| 21 | 17 | 5 |
| 22 | 18 | 5 |
| 23 | null | 6 |
| 24 | null | 6 |
| 25 | 19 | 7 |
| 26 | 20 | 7 |
| 27 | null | 8 |
| 28 | 21 | 9 |

Answer 1

Try this one:
First, this assigns a same ROW_NUMBER for continuous Non-NULL MyMarker. ROW_NUMBER is NULL for NULL MyMarkers. After that, you want to add a ROW_NUMBER for NULL MyMarkers such that the value is between the previous NON-NULL and the next NON-NULL. Then use DENSE_RANK to finally assign MyGroupNumber:
SQL Fiddle
;WITH Cte AS(
SELECT *,
RN = ROW_NUMBER() OVER(ORDER BY MyOrdinal) - MyMarker + 1
FROM #Holder
),
CteApply AS(
SELECT
t.MyOrdinal,
t.MyMarker,
MyGroupNumber =
CASE
WHEN RN IS NULL THEN x.NewRN
ELSE RN
END
FROM Cte t
OUTER APPLY(
SELECT TOP 1 RN * 1.1 AS NewRN
FROM Cte
WHERE
t.MyOrdinal > MyOrdinal
AND MyMarker IS NOT NULL
ORDER BY MyOrdinal DESC
)x
)
SELECT
MyOrdinal,
MyMarker,
MyGroupNumber = DENSE_RANK() OVER(ORDER BY MyGroupNumber)
FROM CteApply

Answer 2

For Sql Server 2012:
select *, sum(b) over(order by myordinal)
from(select *,
case when (lag(mymarker) over(order by myordinal) is not null
and mymarker is null) or
(lag(mymarker) over(order by myordinal) is null
and mymarker is not null)
then 1 else 0 end as b
from #Holder) t
First you mark rows with 1 where there is a change from null to not null or from not null to null. Other columns are marked as 0. Then running sum of all rows till current.
Fiddle http://sqlfiddle.com/#!6/9eecb/5015
For Sql Server 2008:
with cte1 as (select *,
case when (select max(enddate) from t ti
where ti.ruleid = t.ruleid and ti.startdate < t.startdate) = startdate
then 0 else 1 end as b
from t),
cte2 as(select *, sum(b) over(partition by ruleid order by startdate) as s
from cte1)
select RuleID,
Name,
min(startdate),
case when count(*) = count(enddate)
then max(enddate) else null end from cte2
group by s, ruleid, name
Fiddle http://sqlfiddle.com/#!6/4191d/6