SELECT ProductSales.* /*Don't use SELECT * in Production Code!*/
FROM   ProductSales
JOIN   ( SELECT   CustomerID
         FROM     ProductSales
         WHERE    ProductID = 'xyz'
         GROUP BY CustomerID /*Returns just one record per customer*/
        ) AS [ProductOwners]
  ON   ProductSales.CustomerID = ProductOwners.CustomerID;

WITH [ProductOwners]
AS
( SELECT   CustomerID
  FROM     ProductSales
  WHERE    ProductID = 'xyz'
  GROUP BY CustomerID /*Returns just one record per customer*/
 )
SELECT ProductSales.*
FROM   ProductSales
JOIN   ProductOwners
  ON   ProductSales.CustomerID = ProductOwners.CustomerID;

DECLARE @ProductOwners AS TABLE
( CustomerID  INT  PRIMARY KEY );

INSERT @ProductOwners
SELECT   CustomerID
FROM     ProductSales
WHERE    ProductID = 'xyz'
GROUP BY CustomerID; /*Returns just one record per customer*/

SELECT ProductSales.*
FROM   ProductSales
JOIN   @ProductOwners AS [ProductOwners]
  ON   ProductSales.CustomerID = ProductOwners.CustomerID;

CREATE TABLE #ProductOwners
( CustomerID  INT  PRIMARY KEY );

INSERT   #ProductOwners
SELECT   CustomerID
FROM     ProductSales
WHERE    ProductID = 'xyz'
GROUP BY CustomerID; /*Returns just one record per customer*/

SELECT ProductSales.*
FROM   ProductSales
JOIN   #ProductOwners AS [ProductOwners]
  ON   ProductSales.CustomerID = ProductOwners.CustomerID;

IF OBJECT_ID('TempDB..#StudentDetails') IS NOT NULL
  DROP TABLE #StudentDetails;

CREATE TABLE #StudentDetails
( StudentID    INT          PRIMARY KEY
 ,StudentName  VARCHAR(15)  NOT NULL
 );

INSERT #StudentDetails
VALUES(1,'SMITH');
 
INSERT #StudentDetails
VALUES(2,'ALLEN');
 
INSERT #StudentDetails
VALUES(3,'JONES');

INSERT #StudentDetails
VALUES(4,'MARTIN');

INSERT #StudentDetails
VALUES(5,'JAMES');
GO

IF OBJECT_ID('TempDB..#StudentTotalMarks') IS NOT NULL
  DROP TABLE #StudentTotalMarks;

CREATE TABLE #StudentTotalMarks
( StudentID    INT  PRIMARY KEY
 ,StudentMarks INT  NOT NULL
 );

INSERT INTO #StudentTotalMarks
VALUES(1,230);

INSERT INTO #StudentTotalMarks
VALUES(2,255);

INSERT INTO #StudentTotalMarks
VALUES(3,200);

DELETE #StudentTotalMarks
WHERE  StudentMarks > 250;

UPDATE #StudentTotalMarks
SET    StudentMarks = StudentMarks + 25
FROM   #StudentDetails
JOIN   #StudentTotalMarks
  ON   #StudentDetails.StudentID = #StudentTotalMarks.StudentID;

INSERT    #StudentTotalMarks
        ( StudentID
         ,StudentMarks
         )
SELECT    #StudentDetails.StudentID
         ,25
FROM      #StudentDetails
LEFT JOIN #StudentTotalMarks
       ON #StudentDetails.StudentID = #StudentTotalMarks.StudentID
WHERE     #StudentTotalMarks.StudentID IS NULL;

StudentID   StudentMarks
----------- ------------
1           230
2           255
3           200

StudentID   StudentMarks
----------- ------------
1           255
2           25
3           225
4           25
5           25

MERGE [Target_Table]
USING [Source]
   ON Join_Condition
WHEN MATCHED
 THEN [Action];

MERGE #StudentTotalMarks AS StudentTotalMarks
USING ( SELECT StudentID
               ,StudentName 
         FROM   #StudentDetails
        ) AS StudentDetails
   ON StudentTotalMarks.StudentID = StudentDetails.StudentID
WHEN MATCHED AND StudentTotalMarks.StudentMarks > 250 
  THEN DELETE
WHEN MATCHED
  THEN UPDATE 
       SET StudentTotalMarks.StudentMarks = StudentTotalMarks.StudentMarks + 25
WHEN NOT MATCHED 
  THEN INSERT(StudentID,StudentMarks)
       VALUES(StudentDetails.StudentID,25);

StudentID   StudentMarks
----------- ------------
1           255
3           225
4           25
5           25