The SQL Server Merge statement with examples and syntax.

MERGE dbo.TargetTable tgt   -- Target Table
USING dbo.SourceTable src   -- Source Table
ON tgt.ID = src.ID          -- Main comparison criteria
WHEN MATCHED                -- When ID's exist in both tables
    -- DO SOMETHING
WHEN NOT MATCHED            -- When ID's from Source do not exist in Target
    -- DO SOMETHING
WHEN NOT MATCHED BY SOURCE  -- When ID's from Target do not exist in Source
    -- DO SOMETHING

-- ###
-- ### 
-- ###

---------------------------------------------------------------------
-- Merging Data (https://github.com/srihari-sridharan/TSQLSamples)
---------------------------------------------------------------------

-- Listing 8-2 Code that Creates and Populates Customers and CustomersStage
USE tempdb;

IF OBJECT_ID('dbo.Customers', 'U') IS NOT NULL DROP TABLE dbo.Customers;
GO

CREATE TABLE dbo.Customers
(
  custid      INT         NOT NULL,
  companyname VARCHAR(25) NOT NULL,
  phone       VARCHAR(20) NOT NULL,
  address     VARCHAR(50) NOT NULL,
  CONSTRAINT PK_Customers PRIMARY KEY(custid)
);

INSERT INTO dbo.Customers(custid, companyname, phone, address)
VALUES
  (1, 'cust 1', '(111) 111-1111', 'address 1'),
  (2, 'cust 2', '(222) 222-2222', 'address 2'),
  (3, 'cust 3', '(333) 333-3333', 'address 3'),
  (4, 'cust 4', '(444) 444-4444', 'address 4'),
  (5, 'cust 5', '(555) 555-5555', 'address 5');

IF OBJECT_ID('dbo.CustomersStage', 'U') IS NOT NULL DROP TABLE dbo.CustomersStage;
GO

CREATE TABLE dbo.CustomersStage
(
  custid      INT         NOT NULL,
  companyname VARCHAR(25) NOT NULL,
  phone       VARCHAR(20) NOT NULL,
  address     VARCHAR(50) NOT NULL,
  CONSTRAINT PK_CustomersStage PRIMARY KEY(custid)
);

INSERT INTO dbo.CustomersStage(custid, companyname, phone, address)
VALUES
  (2, 'AAAAA', '(222) 222-2222', 'address 2'),
  (3, 'cust 3', '(333) 333-3333', 'address 3'),
  (5, 'BBBBB', 'CCCCC', 'DDDDD'),
  (6, 'cust 6 (new)', '(666) 666-6666', 'address 6'),
  (7, 'cust 7 (new)', '(777) 777-7777', 'address 7');

-- Query tables
SELECT * FROM dbo.Customers;

SELECT * FROM dbo.CustomersStage;

-- MERGE Example 1: Update existing, add missing
MERGE INTO dbo.Customers AS TGT
USING dbo.CustomersStage AS SRC
  ON TGT.custid = SRC.custid
WHEN MATCHED THEN
  UPDATE SET
    TGT.companyname = SRC.companyname,
    TGT.phone = SRC.phone,
    TGT.address = SRC.address
WHEN NOT MATCHED THEN 
  INSERT (custid, companyname, phone, address)
  VALUES (SRC.custid, SRC.companyname, SRC.phone, SRC.address);

-- MERGE Example 2: Update existing, add missing, delete missing in source
MERGE dbo.Customers AS TGT
USING dbo.CustomersStage AS SRC
  ON TGT.custid = SRC.custid
WHEN MATCHED THEN
  UPDATE SET
    TGT.companyname = SRC.companyname,
    TGT.phone = SRC.phone,
    TGT.address = SRC.address
WHEN NOT MATCHED THEN 
  INSERT (custid, companyname, phone, address)
  VALUES (SRC.custid, SRC.companyname, SRC.phone, SRC.address)
WHEN NOT MATCHED BY SOURCE THEN
  DELETE;

-- MERGE Example 3: Update existing that changed, add missing
MERGE dbo.Customers AS TGT
USING dbo.CustomersStage AS SRC
  ON TGT.custid = SRC.custid
WHEN MATCHED AND 
       (   TGT.companyname <> SRC.companyname
        OR TGT.phone       <> SRC.phone
        OR TGT.address     <> SRC.address) THEN
  UPDATE SET
    TGT.companyname = SRC.companyname,
    TGT.phone = SRC.phone,
    TGT.address = SRC.address
WHEN NOT MATCHED THEN 
  INSERT (custid, companyname, phone, address)
  VALUES (SRC.custid, SRC.companyname, SRC.phone, SRC.address);