DROP TABLE Stock;
CREATE TABLE Stock(item_id int UNIQUE, balance int);
INSERT INTO Stock VALUES (10, 2200);
INSERT INTO Stock VALUES (20, 1900);
SELECT * FROM Stock ORDER BY item_id; 

MERGE INTO Stock t
-- USING (VALUES(10,100)) AS s(item_id,balance)
 USING (SELECT 10,100) AS s(item_id,balance)
 ON s.item_id=t.item_id
 WHEN MATCHED THEN UPDATE SET balance=t.balance + s.balance
 WHEN NOT MATCHED THEN INSERT VALUES(s.item_id,s.balance)
 ; 

SELECT * FROM Stock ORDER BY item_id; 

MERGE INTO Stock t
-- USING (VALUES(30,2000)) AS s(item_id,balance)
 USING (SELECT 30,2000) AS s(item_id,balance)
 ON s.item_id=t.item_id
 WHEN MATCHED THEN UPDATE SET balance=t.balance + s.balance
 WHEN NOT MATCHED THEN INSERT VALUES(s.item_id,s.balance)
 ; 

SELECT * FROM Stock ORDER BY item_id;