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

We’ll go back to the AdventureWorks2008 database and look again at the query where we wanted to
know the orders that happened on the first date that an order was placed in the system. However, this
time we want to add a new twist: We want to know the SalesOrderID(s) and OrderDate of the first
order in the system for each customer. That is, we want to know the first day that a customer placed an
order and the IDs of those orders. Let’s look at it piece by piece.
First, we want the OrderDate, SalesOrderID, and CustomerID for each of our results. All of that information
can be found in the SalesOrderHeader table, so we know that our query is going to be based, at
least in part, on that table.
Next, we need to know what the first date in the system was for each customer. That’s where the tricky
part comes in. When we did this with a nested subquery, we were only looking for the first date in the
entire file — now we need a value that’s by individual customer.
This wouldn’t be that big a deal if we were to do it in two separate queries — we could just create a temporary
table and then join back to it.
A temporary table is pretty much just what it sounds like — a table that is created for temporary use
and will go away after our processing is complete. Exactly how long it will stay around is variable and
is outside the scope of this chapter. We will, however, visit temporary tables a bit more as we continue
through the book.
The temporary table solution might look something like this:
USE AdventureWorks2008;
-- Get a list of customers and the date of their first order
SELECT soh.CustomerID, MIN(soh.OrderDate) AS OrderDate
INTO #MinOrderDates
FROM Sales.SalesOrderHeader soh
GROUP BY soh.CustomerID;
-- Do something additional with that information
SELECT soh.CustomerID, soh.SalesOrderID, soh.OrderDate
FROM Sales.SalesOrderHeader soh
JOIN #MinOrderDates t
ON soh.CustomerID = t.CustomerID
AND soh.OrderDate = t.OrderDate
ORDER BY soh.CustomerID;
DROP TABLE #MinOrderDates;
We get back a little over 19,000 rows:
(19119 row(s) affected)
CustomerID SalesOrderID OrderDate
----------- ------------ -----------------------
11000 43793 2001-07-22 00:00:00.000
11001 43767 2001-07-18 00:00:00.000
11002 43736 2001-07-10 00:00:00.000
…
Chapter 7: Adding More to Our Queries
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