How Can I Efficiently Calculate a Running Total of Distinct Values in SQL Server Using Partitions?

Achieving Distinct Count Using Partitions and Dense_Rank() OVER

When attempting to calculate a running total of distinct values using COUNT() OVER, the inclusion of the DISTINCT keyword often results in errors. While you could resort to a traditional approach like correlated subqueries, a more efficient solution exists.

In SQL Server, the DISTINCT keyword is not directly supported within partition functions. However, by utilizing the dense_rank() function, you can obtain a similar result effectively. The following formula will return the desired count for distinct values:

dense_rank() over (partition by [Mth] order by [UserAccountKey]) 
+ dense_rank() over (partition by [Mth] order by [UserAccountKey] desc) 
- 1

This formula calculates the dense rank of each UserAccountKey in ascending and descending order within each month and subsequently subtracts one. The result is precisely the number of unique UserAccountKeys within that month, providing you with the running total you seek.

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