SELECT
	T1.SerialNumber,
	CASE WHEN (DATEADD(MONTH, T2.Schedule, T2.Date1) >= CAST(CURRENT_TIMESTAMP AS DATE)) THEN 1
		WHEN (T2.Extension = 1 AND T2.DateExtended >= CAST(CURRENT_TIMESTAMP AS DATE)) THEN 1
		WHEN (DATEADD(MONTH, T2.Schedule, T2.Date1) < CAST(CURRENT_TIMESTAMP AS DATE) AND (T2.DateExtended IS NULL OR T2.DateExtended < CAST(CURRENT_TIMESTAMP AS DATE))) THEN 0
		ELSE CAST(NULL AS TINYINT) END AS DetermineIsActive,
	CAST (CURRENT_TIMESTAMP AS DATE) AS CurrentTimestamp
FROM
	dbo.Table1 AS T1
		INNER JOIN dbo.Table2 AS T2
			ON T1.SerialNumber = T2.SerialNumber

  MERGE dbo.Table1 T1
  USING (SELECT SerialNumber, DetermineIsActive, CurrentTimestamp FROM vIsActiveUpdate) AS vT2
  ON T1.SerialNumber = vT2.SerialNumber
  WHEN MATCHED THEN UPDATE
	SET T1.IsActive = vT2.DetermineIsActive;

We walk in the dark places no others will enter
We stand on the bridge and no one may pass