qr code generator c# Lesson 2: Implementing Subqueries in C#

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Lesson 2: Implementing Subqueries
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INNER JOIN (SELECT CustomerID, DATEADD(dd, -1, OrderDate) AS OrderDate FROM #orderdates tbl1 WHERE NOT EXISTS(SELECT * FROM #orderdates tbl2 WHERE DATEDIFF(dd, tbl2.OrderDate, tbl1.OrderDate) = 1 AND tbl1.CustomerID = tbl2.CustomerID) AND OrderDate <> (SELECT MIN(OrderDate) FROM #orderdates)) t2 ON t1.OrderDate <= t2.OrderDate AND t1.CustomerID = t2.CustomerID GROUP BY t1.CustomerID, t1.OrderDate) a ORDER BY CustomerID, StartGap;
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Lesson Summary
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Noncorrelated subqueries are independent queries that are embedded within an outer query and are used to retrieve a scalar value or list of values that can be consumed by the outer query to make code more dynamic. Correlated subqueries are queries that are embedded within an outer query but reference values within the outer query.
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Using Additional Query Techniques
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Lesson 3: applying Ranking Functions
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Ranking functions are used to provide simple analytics such as statistical ordering or segmentation. In this lesson, you learn how to use the four ranking functions that ship with SQL Server 2008.
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After this lesson, you will be able to:
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Use ranking functions in your queries.
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Estimated lesson time: 20 minutes
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Ranking Data
T-SQL has four functions that can be used for ranking data: ROW_NUMBER, RANK, DENSE_ RANK, and NTILE. The ROW_NUMBER function assigns a number from 1 to n based on a user-specified sorting order. ROW_NUMBER does not account for ties within the result set, so if you have rows with the same values within the column(s) that you are ordering by, repeated calls to the database for the same result set can produce different row numbering. The following example returns the salesperson along with his or her year-to-date sales and is numbered in descending order according to the year-to-date sales amount:
SELECT p.FirstName, p.LastName, ROW_NUMBER() OVER(ORDER BY s.SalesYTD DESC) AS 'RowNumber', s.SalesYTD, a.PostalCode FROM Sales.SalesPerson s INNER JOIN Person.Person p ON s.BusinessEntityID = p.BusinessEntityID INNER JOIN Person.BusinessEntityAddress ba ON p.BusinessEntityID = ba.BusinessEntityID INNER JOIN Person.Address a ON a.AddressID = ba.AddressID WHERE s.TerritoryID IS NOT NULL
You can also use the ROW_NUMBER function with an aggregate to provide a sequence number within each group. You generate a number within each group by providing an optional PARTITION BY clause as follows:
SELECT p.FirstName, p.LastName, ROW_NUMBER() OVER (PARTITION BY s.TerritoryID ORDER BY SalesYTD DESC) AS 'RowNumber', s.SalesYTD, s.TerritoryID FROM Sales.SalesPerson s INNER JOIN Person.Person p ON s.BusinessEntityID = p.BusinessEntityID INNER JOIN Person.BusinessEntityAddress ba ON p.BusinessEntityID = ba.BusinessEntityID INNER JOIN Person.Address a ON a.AddressID = ba.AddressID WHERE s.TerritoryID IS NOT NULL
If you need to number a result set but also deal with ties, you can use the RANK function. If the result set does not have any ties, RANK produces the same results as ROW_NUMBER.
Lesson 3: Applying Ranking Functions CHAPTER 4 133
However, if there are ties, RANK assigns the same value to each row that is tied and then skips to the next value, leaving a gap in the sequence corresponding to the number of rows that were tied. The following examples show how RANK is applied to duplicates as well as within each aggregate grouping:
SELECT a.ProductID, b.Name, a.LocationID, a.Quantity, RANK() OVER (PARTITION BY a.LocationID ORDER BY a.Quantity DESC) AS 'Rank' FROM Production.ProductInventory a INNER JOIN Production.Product b ON a.ProductID = b.ProductID ORDER BY b.Name
SELECT a.ProductID, b.Name, a.LocationID, a.Quantity, RANK() OVER (PARTITION BY a.LocationID ORDER BY a.Quantity DESC) AS 'Rank' FROM Production.ProductInventory a INNER JOIN Production.Product b ON a.ProductID = b.ProductID ORDER BY 'Rank'
If you do not want any gaps in a sequence, you can use the DENSE_RANK function. DENSE_ RANK assigns the same value to each duplicate but does not produce gaps in the sequence. The following two examples show the same result set when DENSE_RANK is applied:
SELECT a.ProductID, b.Name, a.LocationID, a.Quantity, DENSE_RANK() OVER (PARTITION BY a.LocationID ORDER BY a.Quantity DESC) AS 'DenseRank' FROM Production.ProductInventory a INNER JOIN Production.Product b ON a.ProductID = b.ProductID ORDER BY b.Name
SELECT a.ProductID, b.Name, a.LocationID, a.Quantity, DENSE_RANK() OVER (PARTITION BY a.LocationID ORDER BY a.Quantity DESC) AS DenseRank FROM Production.ProductInventory a INNER JOIN Production.Product b ON a.ProductID = b.ProductID ORDER BY DenseRank
NTILE is used to divide a result set into approximately equal groups. For example, if you wanted to split a result set into six groups with approximately the same number of rows in each group, you could use NTILE(6). The following examples show how NTILE can be used to segment a result set:
SELECT p.FirstName, p.LastName, NTILE(4) OVER(ORDER BY s.SalesYTD DESC) AS QuarterGroup, s.SalesYTD, a.PostalCode FROM Sales.SalesPerson s INNER JOIN Person.Person p ON s.BusinessEntityID = p.BusinessEntityID INNER JOIN Person.BusinessEntityAddress ba ON p.BusinessEntityID = ba.BusinessEntityID INNER JOIN Person.Address a ON a.AddressID = ba.AddressID WHERE s.TerritoryID IS NOT NULL
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