SQL Server: Distinct Count with Conditional Aggregation Not Returning Expected Results
I've been banging my head against this for hours. I tried several approaches but none seem to work. I'm working on SQL Server 2019 and trying to get a distinct count of users who made purchases in different regions, but my query is returning unexpected results. The goal is to count unique user IDs for each region, but my current approach seems to be miscounting. Here's my query: ```sql SELECT Region, COUNT(DISTINCT UserID) AS UniqueUserCount FROM Purchases WHERE PurchaseDate >= '2023-01-01' GROUP BY Region; ``` I expected to see the unique counts segmented by region, but I noticed that the counts are inflated, especially in regions where the same users are making multiple purchases. After reviewing, I realized that using `COUNT(DISTINCT UserID)` may not work as I intended when combined with other filters I'm planning to add later. For instance, I want to include a condition that filters out purchases below a certain amount: ```sql SELECT Region, COUNT(DISTINCT UserID) AS UniqueUserCount FROM Purchases WHERE PurchaseDate >= '2023-01-01' AND Amount >= 100 GROUP BY Region; ``` Even with that filter, the counts still seem off. I've also tried using a subquery to first filter the purchases and then count distinct users, but it doesn't seem to help either: ```sql SELECT Region, COUNT(DISTINCT UserID) AS UniqueUserCount FROM (SELECT UserID, Region FROM Purchases WHERE Amount >= 100) AS FilteredPurchases GROUP BY Region; ``` No matter the approach I take, I'm worried that the way SQL handles distinct counts in this context might be leading to confusion. Are there any best practices to ensure I get the correct distinct counts, or is there a better way to structure my query? Any insights would be greatly appreciated! My development environment is Linux. For reference, this is a production web app.