SQL Server Count Consecutive Preceding Rows based on Values

Question

This is difficult question for me to write out.

I have a table like below:

Date	ItemNumber	FlagA	FlagB
2020-01-01	101	Y	N
2020-01-02	101	N	N
2020-01-03	101	Y	N
2020-01-04	101	Y	N
2020-01-05	101	Y	Y
2020-01-01	102	Y	N
2020-01-02	102	N	N
2020-01-03	102	N	N
2020-01-04	102	Y	Y

My goal is to count the consecutive dates where FlagA = Y including and preceding the date where FlagB = Y for each ItemNumber. The last date for each ItemNumber in the table will always have FlagB = Y.

I'm trying to achieve this by adding an additional column:

Date	ItemNumber	FlagA	FlagB	RunningCount
2020-01-01	101	Y	N	0
2020-01-02	101	N	N	0
2020-01-03	101	Y	N	1
2020-01-04	101	Y	N	2
2020-01-05	101	Y	Y	3
2020-01-01	102	Y	N	0
2020-01-02	102	N	N	0
2020-01-03	102	N	N	0
2020-01-04	102	Y	Y	1

I'm on SQL Server 2012 but do not have a lot of experience with window functions. I've tried several things including:

COUNT(CASE WHEN [FLagB] = 'Y' THEN 1 ELSE 0 END) 
              over (partition by [ItemNumber],[FlagB] 
                   order by [Date] ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS [RunningCount]

and:

COUNT(CASE WHEN [FLagB] = 'Y' THEN [ItemNumber] END) 
              over (PARTITION BY [ItemNumber] 
                   order by [Date] ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS [RunningCount]

but these haven't gotten correct results.

There may be a better way to go about this - I don't need to add the new column to the table, I only need a query that provides the count for each ItemNumber.

Any advice would be greatly appreciated. Thank you in advance.

Answer 1

You seem to want an enumeration of flagA = Y for the most recent uninterrupted sequence. That would be:

select t.*,
       (case when grp = 0 and flagA = 'Y'
             then row_number() over (partition by grp, flagA order by date) 
             else 0
        end) as runningCount
from (select t.*,
             sum(case when flagA = 'N' then 1 else 0 end) over (partition by itemNumber order by date desc) as grp
      from t
     ) t;