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Chapter 7: Adding More to Our Queries
But let’s say, just for instance, that we are regularly purging data from the system, and we still want to
ask this same question as part of an automated report.
Since it’s going to be automated, we can’t run a query to find out what the first date in the system is and
manually plug that into our query — or can we? Actually, the answer is: “Yes, we can,” by putting it all
into just one statement:
SELECT DISTINCT soh.OrderDate, sod.ProductID
FROM Sales.SalesOrderHeader soh
JOIN Sales.SalesOrderDetail sod
ON soh.SalesOrderID = sod.SalesOrderID
WHERE OrderDate = (SELECT MIN(OrderDate) FROM Sales.SalesOrderHeader);
It’s just that quick and easy. The inner query (SELECT MIN...) retrieves a single value for use in the
outer query. Since we’re using an equal sign, the inner query absolutely must return only one column
from one single row or you will get a runtime error.
Notice that I added the order date to this new query. While it did not have to be there for the query to
report the appropriate ProductID's, it does clarify what date those ProductID's are from. Under the
first query, we knew what date because we had explicitly said it, but under this new query, the date is
data driven, so it is often worthwhile to provide it as part of the result.
Nested Queries Using Subqueries That Return Multiple Values
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Perhaps the most common of all subqueries that are implemented in the world are those that retrieve
some form of domain list and use it as criteria for a query.
For this one, let’s revisit a problem we looked at in Chapter 4 when we were examining outer joins.
What you want is a list of all the products that have special offers.
We might write something like this:
SELECT ProductID, Name
FROM Production.Product
WHERE ProductID IN (
SELECT ProductID FROM Sales.SpecialOfferProduct);
This returns 295 rows:
ProductID Name
----------- --------------------------------------------------
680 HL Road Frame - Black, 58
706 HL Road Frame - Red, 58
707 Sport-100 Helmet, Red
…
…
997 Road-750 Black, 44
998 Road-750 Black, 48
999 Road-750 Black, 52
(295 row(s) affected)