how to generate and print barcode in c# windows application Lesson 1: Tuning Queries in C#.NET

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Lesson 1: Tuning Queries
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If these queries had been written with an outer join, they would not be syntactically equal and could have substantially different performance.
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Subqueries Without Correlation to the Outer Query
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As explained in 4, Using Additional Query Techniques, an uncorrelated subquery is executed only once per query execution and returns only one value. These queries typically incur very little overhead. Note that this type of subquery cannot have any reference (correlation) to the outer query. The following example uses a subquery to return all products that are cheaper than the average product price. The subquery calculating the average product price is executed first (only once), and then the value returned by the subquery is used as a parameter in the outer query:
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USE AdventureWorks;
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SELECT p.ProductID ,p.Name ,p.ListPrice FROM Production.Product AS p WHERE p.ListPrice > ( SELECT AVG(p2.ListPrice) FROM Production.Product AS p2 );
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Correlated Subqueries
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Correlated subqueries include a reference to the outer query. Typically, this reference is used to filter the correlated subquery. A correlated subquery is typically equal in performance compared to using a JOIN when used in combination with the EXISTS operator to filter the outer query. The following example query uses the EXISTS operator to return only products that have been sold:
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USE AdventureWorks;
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SELECT p.ProductID, p.Name FROM Production.Product AS p WHERE EXISTS ( SELECT * FROM Sales.SalesOrderDetail AS sod WHERE sod.ProductID = p.ProductID );
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While this type of correlated subquery is typically a good implementation, the use of correlated subqueries in the SELECT clause often has a negative effect on performance compared to JOINs. Of course, this depends on the number of rows returned by the outer query. If a large number of rows are returned, each query in the SELECT clause would be executed for each row, so that means a large number of query executions. The following
Techniques to Improve Query Performance
query returns 6,224 rows and includes two correlated subqueries. Each of these queries is executed once per row, resulting in a total of 12,448 subquery executions:
USE AdventureWorks;
SELECT soh.SalesOrderID ,soh.OrderDate ,( SELECT TOP(1) sod1.UnitPrice FROM Sales.SalesOrderDetail AS sod1 WHERE sod1.SalesOrderID = soh.SalesOrderID ORDER BY sod1.OrderQty DESC ) AS UnitPrice ,( SELECT TOP(1) sod2.OrderQty FROM Sales.SalesOrderDetail AS sod2 WHERE sod2.SalesOrderID = soh.SalesOrderID ORDER BY sod2.OrderQty DESC ) AS OrderQty FROM Sales.SalesOrderHeader AS soh WHERE soh.TerritoryID = 4;
There is also a potential bug in this query. Because each subquery is executed separately, they might end up using different indexes. This means that these queries might not return values from the same row (which they are probably intended to) if the same value for OrderQty exists for multiple sales order details in any sales order. There are several ways to rewrite this query; the most common one in SQL Server 2008 is probably to use the new APPLY clause. If the subquery is used in the FROM, JOIN, or APPLY clauses, it might also be referred to as a derived table, as explained in 4. The APPLY clause basically gives you the opportunity to combine two subqueries into one, cutting the number of subquery executions in half. For the new query to return the same results as the previous query, you must use an OUTER APPLY. (An OUTER APPLY works similarly to a left outer join, and its counterpart, the CROSS APPLY clause, behaves like an inner join.) This works in this example because, in the previous query, the outer query returns a row even if the subqueries return nothing. The new query could be written as follows:
USE AdventureWorks;
SELECT soh.SalesOrderID ,soh.OrderDate ,a.*
Lesson 1: Tuning Queries
FROM Sales.SalesOrderHeader AS soh OUTER APPLY ( SELECT TOP(1) sod.UnitPrice ,sod.OrderQty FROM Sales.SalesOrderDetail AS sod WHERE sod.SalesOrderID = soh.SalesOrderID ORDER BY sod.OrderQty DESC ) AS a WHERE soh.TerritoryID = 4;
This query has a cost of roughly 76, while the first query s cost was double that, about 151. Another solution to this type of problem is to make use of the ROW_NUMBER function instead of a correlated subquery. By using the ROW_NUMBER function, you can find the specific number of rows that you need by filtering on the row number rather than using the TOP clause. To be able to filter on the result of the ROW_NUMBER function, the query needs to be placed inside a derived table or a common table expression (CTE). The larger the result set, the better this approach performs compared to the previous queries. The cost for the following query drops from 76 to about 3.6, an enormous reduction:
-- Common table expression. WITH a AS ( SELECT soh.SalesOrderID ,soh.OrderDate ,sod.UnitPrice ,sod.OrderQty ,ROW_NUMBER() OVER ( PARTITION BY soh.SalesOrderID ORDER BY sod.OrderQty DESC ) AS RowNo FROM Sales.SalesOrderDetail AS sod INNER JOIN Sales.SalesOrderHeader AS soh ON sod.SalesOrderID = soh.SalesOrderID WHERE soh.TerritoryID = 4 ) SELECT a.SalesOrderID ,a.OrderDate ,a.UnitPrice ,a.OrderQty FROM a WHERE a.RowNo = 1;
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