How do you safely test a MERGE statement before running it against real production data?

You can wrap the MERGE statement in a transaction, run it, review the affected row counts and results using the OUTPUT clause, and then roll back the transaction to undo the changes, giving you a safe way to verify the exact behavior of a complex MERGE statement before committing to running it against your actual production data.

BEGIN TRANSACTION;
MERGE INTO Products AS target
USING StagingProducts AS source
ON target.ProductId = source.ProductId
WHEN MATCHED THEN UPDATE SET target.Price = source.Price
OUTPUT $action, inserted.ProductId;
-- Review the output, then decide
ROLLBACK TRANSACTION;