Beginning Microsoft SQL Server 2008 ... - S3 Tech Training

GROUP BY soh.OrderDate, sod.ProductID
) AS s
ON (s.ProductID = smr.ProductID)
WHEN MATCHED THEN
UPDATE SET smr.QtySold = smr.QtySold + s.QtySold
WHEN NOT MATCHED THEN
INSERT (Year, Month, ProductID, QtySold)
VALUES (DATEPART(yy, s.OrderDate),
DATEPART(m, s.OrderDate),
s.ProductID,
s.QtySold);
We update the date we’re running this for (simulating running it on the second day of the month), and
running it should get us 38 rows:
(38 row(s) affected)
But something is different this time — we already had rows in the table that our new batch of sales may have
matched up with. We know we affected 38 rows, but how did we affect them? Rerun the SELECT on our table:
SELECT *
FROM Sales.MonthlyRollup;
Chapter 7: Adding More to Our Queries
And instead of 230 rows (the 192 plus the 38), we only get 194 rows. Indeed, 36 of our 38 rows were
repeat sales and were therefore treated as updates, rather than insertions. Two rows (ProductIDs 882 and
928) were sales of products that had not been previously sold in that month and thus needed to be
inserted as new rows — one pass over the data, but the equivalent of two statements ran.
We could perform similar actions that decide to delete rows based on matched or not matched conditions.
A Brief Look at BY TARGET versus BY SOURCE
In the examples above, we’ve largely ignored the issue of which is the table to be matched when determining
the action to be performed. The default is BY TARGET, and thus all of our examples (which haven’t
used the BY keyword at all) have been analyzed on whether there is or isn’t a match in the target table
(the table named immediately after the MERGE keyword). The comparison, from a matching perspective, is
similar to an outer join. As a join is analyzed, there can be a match on the source side, the target side, or
both. If you have specified BY TARGET (or not used the BY keyword at all since matching by target is the
default), the action (insert, update, or delete) is applied only if the target side of the join has a match.
Likewise, if you have specified BY SOURCE, then the merge action is only applied if the source side of
the join has a match.
Most of the time, you can map a particular merge action to a specific match scenario:
❑ NOT MATCHED [BY TARGET]: This typically maps to a scenario where you are going to be
inserting rows into a table based on data you found in the source.
❑ MATCHED [BY TARGET]: This implies that the row already exists in the target, and thus it is
likely you will be performing an update action on the target table row.
❑ NOT MATCHED BY SOURCE: This is typically utilized to deal with rows that are missing (and likely
deleted) from the source table, and you will usually be deleting the row in the target under this
scenario (though you may also just update the row to set an inactive flag or similar marker).
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