Merge pull request #17661 from julieMSFT/20201022_mergepreview

added Preview note for Synapse

Author: PRMerger12

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

@@ -30,7 +30,10 @@ ms.author: XiaoyuL
 
 [!INCLUDE [SQL Server SQL Database](../../includes/applies-to-version/sql-asdb-asa.md)]
 
-Runs insert, update, or delete operations on a target table from the results of a join with a source table. For example, synchronize two tables by inserting, updating, or deleting rows in one table based on differences found in the other table.  
+Runs insert, update, or delete operations on a target table from the results of a join with a source table. For example, synchronize two tables by inserting, updating, or deleting rows in one table based on differences found in the other table. 
+
+> [!NOTE]
+> MERGE is currently in preview for Azure Synapse Analytics.
   
 **Performance Tip:** The conditional behavior described for the MERGE statement works best when the two tables have a complex mixture of matching characteristics. For example, inserting a row if it doesn't exist, or updating a row if it matches. When simply updating one table based on the rows of another table, improve the performance and scalability with basic INSERT, UPDATE, and DELETE statements. For example: