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

Let’s say, for example, that we wanted a list of the IDs for all of the products that we have sold at
least 30 of in an individual sale (more than 30 at one time). We can easily get that information from the
SalesOrderDetail table with the following query:
SELECT ProductID
FROM Sales.SalesOrderDetail
WHERE OrderQty > 30;
What we get back is one row matching the ProductID for every row in the SalesOrderDetail table
that has an order quantity that is more than 30:
ProductID
-----------
709
863
863
863
863
863
863
863
715
863
...
...
869
869
867
(31 row(s) affected)
While this meets your needs from a technical standpoint, it doesn’t really meet your needs from a reality
standpoint. Look at all those duplicate rows! While we could look through and see which products sold
more than 30 at a time, the number of rows returned and the number of duplicates can quickly become
overwhelming. Like the problems we’ve discussed before, we have an answer. It comes in the form of
the DISTINCT predicate on your SELECT statement.
Try re-running the query with a slight change:
SELECT DISTINCT ProductID
FROM Sales.SalesOrderDetail
WHERE OrderQty > 30;
Now you come up with a true list of the ProductIDs that sold more than 30 at one time:
ProductID
-----------
863
869
709
864
867
715
(6 row(s) affected)
Chapter 3: The Foundation Statements of T-SQL
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