SQL Server 2019

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CREATE SCHEMA sde;
CREATE TABLE sde.SAP_Load_test
(
  Premise_ID bigint,
  Division char(2),
  InstallationType char(4)
);

CREATE TABLE sde.PREMISE_test
(
  Premise_ID bigint NOT NULL UNIQUE,
  Division char(2),
  InstallationType char(4)
);

INSERT sde.SAP_Load_test(Premise_ID,Division,InstallationType)
VALUES(8000004592, 'SW', 'WWTR'),
      (8000004592, 'WT', 'IIRG'),
      (8000004592, 'WT', 'WATR'),
      (8000004593, 'WT', 'IRRG'),
      (8000004594, 'SW', 'WWTR');

INSERT sde.PREMISE_test (Premise_ID,Division,InstallationType)
VALUES(8000004592, NULL, NULL);
6 rows affected

BEGIN TRANSACTION;
 
UPDATE pt WITH (UPDLOCK, SERIALIZABLE) 
  SET Division = s.Division, 
      InstallationType = s.InstallationType
  FROM sde.PREMISE_test AS pt
  INNER JOIN sde.SAP_Load_test AS s
    ON pt.Premise_ID = s.Premise_ID;

INSERT sde.PREMISE_test(Premise_ID, Division, InstallationType)
  SELECT s.Premise_ID, s.Division, s.InstallationType
    FROM sde.SAP_Load_test AS s
    WHERE NOT EXISTS (SELECT 1 FROM sde.PREMISE_test
      WHERE Premise_ID = s.Premise_ID);
 
COMMIT TRANSACTION;
3 rows affected

SELECT Premise_ID,Division,InstallationType FROM sde.PREMISE_test;


Premise_ID
Division
InstallationType


8000004592
SW
WWTR

8000004593
WT
IRRG

8000004594
SW
WWTR