How to implement guide with t-sql merge statement causing unexpected duplicate entries
I'm learning this framework and Can someone help me understand I'm a bit lost with Quick question that's been bugging me - I've been banging my head against this for hours... I'm stuck on something that should probably be simple. I'm working with a frustrating scenario with a T-SQL `MERGE` statement in SQL Server 2019 that seems to be inserting duplicate entries into my target table, even though I expect it to perform an upsert. Here’s the scenario: I have a source table `SourceData` and a target table `TargetData` with a unique constraint on the `ID` column. The `MERGE` statement looks like this: ```sql MERGE INTO TargetData AS target USING SourceData AS source ON target.ID = source.ID WHEN MATCHED THEN UPDATE SET target.Value = source.Value WHEN NOT MATCHED THEN INSERT (ID, Value) VALUES (source.ID, source.Value); ``` After running this query, I noticed that some IDs are appearing multiple times in `TargetData`, which should not happen given the unique constraint. I checked for duplicates in the `SourceData` table, but there are none. The target table is structured as follows: ```sql CREATE TABLE TargetData ( ID INT PRIMARY KEY, Value NVARCHAR(100) ); ``` I also ensured that the `ID` column in `SourceData` matches what’s in `TargetData`. To troubleshoot, I ran the `MERGE` statement with a `RETURNING` clause to see what it was doing, but that didn't provide clarity either. I even tried isolating the data by selecting a smaller dataset from `SourceData`, yet the duplicates still appeared. Could there be an scenario with the way the `MERGE` statement is handling the data, or is there something else that I'm missing? Any insights would be greatly appreciated. I'm working on a CLI tool that needs to handle this. I'd really appreciate any guidance on this. How would you solve this? I'd really appreciate any guidance on this. I appreciate any insights! For context: I'm using Sql on Ubuntu 22.04. Any ideas what could be causing this? This is for a desktop app running on Linux. I'd really appreciate any guidance on this.