问SQL MERGE语句中的UPDATE-no-op

EN

-- Set-up of test data, this would be the persisted table 
DECLARE @target TABLE (id int NOT NULL PRIMARY KEY) ;
INSERT INTO @target (id) SELECT v.id FROM (VALUES (1), (2)) v(id);

-- START OF THE CODE IN QUESTION
-- The result table variable (will be several columns in the end)
DECLARE @result TABLE (id int NOT NULL, new bit NOT NULL) ;

WITH Source AS (
    -- Imagine a fairly expensive, recursive CTE here
    SELECT * FROM (VALUES (1), (3)) AS Source (id)
)
MERGE INTO @target AS Target
    USING Source
    ON Target.id = Source.id
    -- Perform a no-op on the match to get the output record
    WHEN MATCHED THEN 
        UPDATE SET Target.id=Target.id
    WHEN NOT MATCHED BY TARGET THEN
        INSERT (id) VALUES (SOURCE.id)
    -- select the data to be returned - will be more columns
    OUTPUT source.id, CASE WHEN $action='INSERT' THEN CONVERT(bit, 1) ELSE CONVERT(bit, 0) END
      INTO @result ;

-- Select the result
SELECT * FROM @result;