Question

I have source and table destination tables like this

Source:

    Column1 Column2 Column3 Column4

Destination:

    Column1 Column2 Column5 Column6

How can I implement this logic in SQL SERVER

IF SOURCE.Column1 = DESTINATION.Column1 AND SOURCE.Column2 = DESTINATION.Column2
UPDATE DESTINATION Column5 = SOURCE.Column1 + 12 (some other logic)
ELSE
Do something else

The issue is, that I have to deal with billion rows, so what is the best way to implement the above logic

EDIT1

IF SOURCE.Column1 = DESTINATION.Column1 AND SOURCE.Column2 = DESTINATION.Column2
UPDATE DESTINATION Column5 = SOURCE.Column1 + 12 (some other logic)
ELSE
**INSERT SOURCE table row to the destination**

How can I implement the same with merge statement, because I have to consider only two columns

Answer 1

You can UPDATE with JOIN and that would be more performance wise in your case. Like so:

UPDATE d
SET d.Column5 = CASE WHEN some logic THEN s.Column1 + 12 (some other logic)
                     ELSE Something else
                ELSE
FROM DESTINATION d
LEFT JOIN SOURCE s ON s.Column2 = d.Colcumn2;

---

You can use the MERGE statement like so:

MERGE INTO SOURCE AS TGT
USING DESTINATION AS SRC
  ON TGT.Column1 = SRC.Column1 
WHEN MATCHED THEN
  UPDATE SET
    TGT.Column5 = SRC.Column1 + 12 (some other logic)
WHEN NOT MATCHED THEN
  INSERT (Source columns list)
  VALUES (...);

Note that:

• In the INSERT statement there was no INTO TableName, because the name of the target table is already defined in the MERGE clause, which is SOURCE. Thats why I defined SOURCE AS TGT and DESTINATION AS SRC.

• It is mandatory to end the MERGE statement with semicolon. And it is best practice to do so with al sql statements.