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

there are no other roll up records for the month, any sales for the day are just rolled up and inserted. On
the second day, however, we have a different scenario: We need to roll up and insert new records as we did
the first day, but we only need to update existing records (for products that have already sold that month).
Let’s take a look at how MERGE can manage both actions in one step. Before we get going on this, however,
we need to create our roll up table:
USE AdventureWorks2008
CREATE TABLE Sales.MonthlyRollup
(
Year smallint NOT NULL,
Month tinyint NOT NULL,
ProductID int NOT NULL
FOREIGN KEY
REFERENCES Production.Product(ProductID),
QtySold int NOT NULL,
CONSTRAINT PKYearMonthProductID
PRIMARY KEY
(Year, Month, ProductID)
);
This is a pretty simple example of a monthly roll up table making it very easy to get sales totals by product
for a given year and month. To make use of this, however, we need to regularly populate it with
rolled up values from our detail table. To do this, we’ll use MERGE.
First, we need to start by establishing a result set that will figure out from what rows we need to be sourcing
data for our roll up. For purposes of this example, we’ll focus on August of 2003 and start with our query
for the first day of the month:
SELECT soh.OrderDate, sod.ProductID, SUM(sod.OrderQty) AS QtySold
FROM Sales.SalesOrderHeader soh
JOIN Sales.SalesOrderDetail sod
ON soh.SalesOrderID = sod.SalesOrderID
WHERE soh.OrderDate >= ‘2003-08-01’
AND soh.OrderDate < ‘2003-08-02’
GROUP BY soh.OrderDate, sod.ProductID;
This gets us the total sales, by ProductID, for every date in our range (our range just happens to be limited
to one day).
There is a bit of a trap built into how we’ve done this up to this point. I’ve set the GROUP BY to use
the OrderDate, but OrderDate is a datetime data type as opposed to just a date data type. If our orders
were to start coming in with actual times on them, it would mess with our assumption of the orders all
grouping nicely into one date. If this were a production environment, we would want to cast the Order-
Date to a date data type or use DATEPART to assure that the grouping was by day rather than by time.
With this, we’re ready to build our merge:
MERGE Sales.MonthlyRollup AS smr
USING
(
Chapter 7: Adding More to Our Queries
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