Check out example codes for "tsql merge example". It will help you in understanding the concepts better.

Code Example 1

MERGE esqlProductTarget T
USING esqlProductSource S
ON (S.ProductID = T.ProductID)
WHEN MATCHED 
     THEN UPDATE
     SET    T.Name = S.Name,
            T.ProductNumber = S.ProductNumber,
            T.Color = S.Color
WHEN NOT MATCHED BY TARGET
THEN INSERT (ProductID, Name, ProductNumber, Color)
     VALUES (S.ProductID, S.Name, S.ProductNumber, S.Color)
WHEN NOT MATCHED BY SOURCE
THEN DELETE;

Code Example 2

MERGE sales.category t 
    USING sales.category_staging s
ON (s.category_id = t.category_id)
WHEN MATCHED
    THEN UPDATE SET 
        t.category_name = s.category_name,
        t.amount = s.amount
WHEN NOT MATCHED BY TARGET 
    THEN INSERT (category_id, category_name, amount)
         VALUES (s.category_id, s.category_name, s.amount)
WHEN NOT MATCHED BY SOURCE 
    THEN DELETE;

Code Example 3

MERGE dbo.MyTarget targ
USING (SELECT ... FROM dbo.MySource GROUP BY .....) src
ON (targ.Identifier = src.Identifier
    AND targ.Name = src.ConstituentName
    AND targ.Ticker = src.ConstituentTicker
    AND (targ.CUSIP = src.CUSIP OR targ.ISIN = src.ISIN OR targ.SEDOL = src.SEDOL))
WHEN MATCHED THEN
-- update values
;

Code Example 4

MERGE table1
USING (SELECT table3.keycolumn,
              table2.DataColumn1,
              table2.DataColumn2
       FROM table2
       INNER JOIN table3
           ON table2.anotherKey = table3.anotherKey
       WHERE table2.anotherKey = 'A1') tmpTable
ON 
   table1.keyColumn = tmpTable.keyColumn
WHEN MATCHED THEN
       UPDATE
       SET table1.DataColumn1 = tmpTable.DataColumn1
            ,table1.DataColumn2 = tmpTable.DataColumn2;

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