SQL Server 2019

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IF OBJECT_ID(N'T')IS NOT NULL DROP TABLE Tl;

CREATE TABLE T(ID INT NOT NULL, V INT NULL);

INSERT T(ID,V)VALUES(1,1),(2,NULL),(3,-100),(4,0);

SELECT * FROM T;

DECLARE @ID INT=3, @V INT=55;

MERGE T USING(VALUES(@ID,@V))S(ID,V) ON T.ID=S.ID

WHEN MATCHED THEN UPDATE SET V=S.V

WHEN NOT MATCHED THEN INSERT(ID,V)VALUES(S.ID,S.V);

SELECT * FROM T;