How to Calculate a Running Total of Distinct Values in SQL Server Partitions?

PARTITION Function COUNT() OVER with DISTINCT

Using the PARTITION function with COUNT() to calculate a running total of distinct values can be challenging in SQL Server. While the DISTINCT keyword is supported in aggregate functions, it cannot be used within partition functions. This limitation can be frustrating, especially when attempting to calculate running totals of distinct values.

One traditional method for calculating distinct counts is to use a correlated subquery. However, this approach can be inefficient and difficult to maintain.

Fortunately, there is a simple solution using dense_rank():

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

This expression calculates the running total of distinct UserAccountKeys within each month. It works by first ranking the values in ascending order, then ranking them in descending order, and finally subtracting one from the sum of the two ranks. This approach effectively eliminates duplicate values while providing a running count of the distinct values.

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