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Using Additional Query Techniques
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The main benefit of a derived table is the fact that the result set resides entirely in memory, which allows faster data access than if the result set were on a storage device. In almost all cases, any routine that uses a temporary table to store an intermediate result set that is used by a subsequent query can use a derived table instead.
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Running Aggregates
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By combining derived tables with unequal joins, you can calculate a variety of cumulative aggregates. The following query returns a running aggregate of orders for each salesperson:
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SELECT SH3.SalesPersonID, SH3.OrderDate, SH3.DailyTotal, SUM(SH4.DailyTotal) RunningTotal FROM (SELECT SH1.SalesPersonID, SH1.OrderDate, SUM(SH1.TotalDue) DailyTotal FROM Sales.SalesOrderHeader SH1 WHERE SH1.SalesPersonID IS NOT NULL GROUP BY SH1.SalesPersonID, SH1.OrderDate) SH3 INNER JOIN (SELECT SH2.SalesPersonID, SH2.OrderDate, SUM(SH2.TotalDue) DailyTotal FROM Sales.SalesOrderHeader SH2 WHERE SH2.SalesPersonID IS NOT NULL GROUP BY SH2.SalesPersonID, SH2.OrderDate) SH4 ON SH3.SalesPersonID = SH4.SalesPersonID AND SH3.OrderDate >= SH4.OrderDate GROUP BY SH3.SalesPersonID, SH3.OrderDate, SH3.DailyTotal ORDER BY SH3.SalesPersonID, SH3.OrderDate
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The derived tables are used to combine all orders for salespeople who have more than one order on a single day. The join on SalesPersonID ensures that you are accumulating rows for only a single salesperson. The unequal join allows the aggregate to consider only the rows for a salesperson where the order date is earlier than the order date currently being considered within the result set. By adding a HAVING clause, you can expand a running aggregate to encompass a variety of situations, such as displaying a running sales total only for salespeople who have already met their quota or sales aggregated across a sliding window.
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Correlated Subqueries
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In a correlated subquery, the inner query depends upon the values from the outer query. This causes the inner query to be executed repeatedly based on input from the outer query. The following query returns products and their corresponding list price for all products that have been sold:
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SELECT a.ProductID, a.ListPrice FROM Production.Product a WHERE EXISTS (SELECT 1 FROM Sales.SalesOrderDetail b WHERE b.ProductID = a.ProductID)
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Lesson 2: Implementing Subqueries
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Using a join or any other WHERE clause would return the product every time it were sold; however, that would detail how many times a product had sold instead of detailing products that have sold at least once. The EXISTS argument also improves the performance of the query because SQL Server has to find only a single occurrence within the table for the WHERE clause to be true. As soon as a value is located, SQL Server quits looking at the remainder of the rows because the return value would not change from that point forward.
exaM tiP
For the exam, you are going to need to read a query and, based solely on the SELECT statement, determine whether the query can be used to solve the business problem presented.
Quick check
1. What is the difference between a correlated and a noncorrelated subquery 2. What is a derived table
Quick check answers
1. A noncorrelated subquery is a query that is embedded within another query but
does not reference any columns from the outer query. A correlated subquery is embedded within another query and references columns within the outer query.
2. A derived table is a SELECT statement that is embedded within a FROM clause.
Pr actice
Finding Sequence Gaps
In this practice, you use subqueries to find gaps in a sequence.
e xercise
Find Sequence Gaps
In the following exercise, you use subqueries to find gaps in a sequence.
Open a new query window, type and execute the following code to load a table with test data:
CREATE TABLE #orderdates (CustomerID OrderDate INT NOT NULL,
DATETIME NOT NULL);
DECLARE @startDate @endDate @CustomerID
datetime, datetime, int
SET @CustomerID = 1
Using Additional Query Techniques
WHILE @CustomerID < 100 BEGIN SELECT @startDate = 'Jan 01, 2007', @endDate = DATEADD(yy, 1, @startDate);
WITH dates AS (SELECT @startDate AS begindate UNION ALL SELECT DATEADD(dd,1,begindate) FROM dates WHERE begindate < dateadd(dd,-1,@endDate ) )
INSERT INTO #orderdates SELECT @CustomerID, begindate FROM dates OPTION (MAXRECURSION 0);
SET @CustomerID = @CustomerID + 1 END
SELECT * FROM #orderdates;
In the existing query window, type, highlight, and execute the following code to introduce gaps within the sequence:
--Now delete some of the rows to produce gaps DELETE #orderdates WHERE DATEDIFF(dd, 0, OrderDate)%11 = 0 ;
--Produce a couple of multi-day gaps DELETE FROM #orderdates WHERE OrderDate IN ('1/4/2007','2/17/2007') AND CustomerID%3 = 0
SELECT * FROM #orderdates;
In the existing query window, type, highlight, and execute the following code to retrieve a list of all the sequence gaps:
SELECT CustomerID, StartGap, EndGap, DATEDIFF(dd,StartGap,EndGap) + 1 AS NumberMissingDays FROM (SELECT t1.CustomerID, t1.OrderDate AS StartGap, MIN(t2.OrderDate) AS EndGap FROM (SELECT CustomerID, DATEADD(dd, 1, OrderDate) AS OrderDate FROM #orderdates tbl1 WHERE NOT EXISTS(SELECT * FROM #orderdates tbl2 WHERE DATEDIFF(dd, tbl1.OrderDate, tbl2.OrderDate) = 1 AND tbl1.CustomerID = tbl2.CustomerID) AND OrderDate <> (SELECT MAX(OrderDate) FROM #orderdates)) t1
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