Merge pull request #17626 from XiaoyuMSFT/patch-15

Update MERGE with known issue

Author: PRMerger20

docs/t-sql/statements/merge-transact-sql.md

@@ -227,6 +227,7 @@ Specifies the graph match pattern. For more information about the arguments for
 >[!NOTE]
 > In Azure Synapse Analytics, the MERGE command (preview) has following differences compared to SQL server and Azure SQL database.  
 > - A MERGE update is implemented as a delete and insert pair. The affected row count for a MERGE update includes the deleted and inserted rows. 
+> - During the preview, the MERGE command does not work with tables with UNIQUE constraints.  This will be fixed in a later release soon.
 > - The support for tables with different distribution types is described in this table:
 
 >|MERGE CLAUSE in Azure Synapse Analytics|Supported TARGE distribution table| Supported SOURCE distribution table|Comment|