SQL Server 2017: Unintentional Duplicates in Merge Statement During Upsert Operation
I keep running into I'm converting an old project and I just started working with I'm working with unexpected behavior while using the `MERGE` statement in SQL Server 2017 for an upsert operation on my `Users` table... The purpose of this `MERGE` is to insert new records or update existing ones based on a unique identifier, `UserID`. However, I'm seeing duplicated entries for some users in the `Users` table when the `MERGE` statement is executed. The `Users` table has a unique constraint on `UserID`, yet the duplicates are still being inserted under certain conditions. Here is the basic structure of my `Users` table: ```sql CREATE TABLE Users ( UserID INT PRIMARY KEY, UserName NVARCHAR(100), Email NVARCHAR(100) ); ``` And here’s the `MERGE` statement I’m using: ```sql MERGE INTO Users AS target USING (SELECT UserID, UserName, Email FROM StagingUsers) AS source ON target.UserID = source.UserID WHEN MATCHED THEN UPDATE SET UserName = source.UserName, Email = source.Email WHEN NOT MATCHED THEN INSERT (UserID, UserName, Email) VALUES (source.UserID, source.UserName, source.Email); ``` The source data comes from a temporary table `StagingUsers`. I’ve verified that `StagingUsers` does not contain duplicates for `UserID`. However, there are scenarios where the `MERGE` seems to insert a duplicate row after running the statement multiple times. I receive the following behavior when attempting to insert a duplicate: ``` Violation of PRIMARY KEY constraint 'PK_Users'. want to insert duplicate key in object 'dbo.Users'. ``` To troubleshoot, I checked for race conditions since this operation might be executed concurrently by different processes. I also consistently use transactions around my `MERGE` statement to ensure atomicity, but the scenario continues. I've tried simplifying the `USING` clause by directly providing values instead of a subquery, and I've also ensured that the `UserID` in `StagingUsers` is indeed unique. Is there any specific behavior of the `MERGE` statement in SQL Server that might lead to this situation, or is there a potential scenario with my data handling before the `MERGE` execution? Any insights or advice would be greatly appreciated! My development environment is Linux. I've been using Sql for about a year now. I recently upgraded to Sql 3.10. For context: I'm using Sql on Windows 10. Cheers for any assistance! My development environment is Windows 10.