asp.net mvc qr code generator Figure 5-1. Execution plan for obtaining aggregates with subqueries in .NET framework

Creator QR-Code in .NET framework Figure 5-1. Execution plan for obtaining aggregates with subqueries

Figure 5-1. Execution plan for obtaining aggregates with subqueries
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You will notice that the index I created on the qty column is scanned twiceonce to calculate the sum, and once to calculate the average. In other words, provided that you have an index on the aggregated column, the index will be scanned once for each subquery that returns an aggregate. If you don't have an index containing the aggregated column, matters are even worse, as you'll get a table scan for each subquery. This query can be optimized using a key technique that utilizes a cross join. You can calculate all needed aggregates in one query that will require only a single index or table scan. Such a query will produce a single result row with all aggregates. You create a derived table defined by this query and cross it with the base table. Now you have access to both attributes and aggregates. Here's the solution query, which produces the more optimal plan shown in Figure 5-2: SELECT stor_id, ord_num, title_id, CONVERT(VARCHAR(10), ord_date, 120) AS ord_date, qty, CAST(1.*qty / sumqty * 100 AS DECIMAL(5, 2)) AS per, qty - avgqty as diff FROM dbo.sales, (SELECT SUM(qty) AS sumqty, AVG(qty) AS avgqty FROM dbo.sales) AS AGG;
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Figure 5-2. Execution plan for obtaining aggregates with a cross join
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As you can see in the plan, the index on the qty column is scanned only once, and both aggregates are calculated with the same scan. Of course, in SQL Server 2005 you can use a common table expression (CTE), which you might find easier to read:
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WITH Agg AS ( SELECT SUM(qty) AS sumqty, AVG(qty) AS avgqty FROM dbo.sales ) SELECT stor_id, ord_num, title_id, CONVERT(VARCHAR(10), ord_date, 120) AS ord_date, qty, CAST(1.*qty / sumqty * 100 AS DECIMAL(5, 2)) AS per, qty - avgqty as diff FROM dbo.sales, Agg;
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You will find that both queries generate the same plan. In 6, I'll demonstrate how to use the new OVER clause in SQL Server 2005 to tackle similar problems. Once you're done experimenting with this technique, drop the index on the qty column: DROP INDEX dbo.sales.idx_qty;
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Inner joins are used to match rows between two tables based on some criterion. Out of the first three query logical processing phases, inner joins apply the first twonamely, Cartesian product and ON filter. There's no phase that adds outer rows. Consequently, if an INNER JOIN query contains both an ON clause and a WHERE clause, logically they are applied one after the other. With one exception, there's no difference between specifying a logical expression in the ON clause or in the WHERE clause of an INNER JOIN, because there's no intermediate step that adds outer rows between the two. The one exception is when you specify GROUP BY ALL. Remember that GROUP BY ALL adds back groups that were filtered out by the WHERE clause, but it does not add back groups that were filtered out by the ON clause. Remember also that this is a nonstandard legacy feature that you should avoid using. As for performance, when not using the GROUP BY ALL option, you will typically get the same plan regardless of where you place the filter expression. That's because the optimizer is aware that there's no difference. I should always be cautious when saying such things related to optimization choices because the process is so dynamic. As for the two supported join syntaxes, using the ANSI SQL:1992 syntax, you have more flexibility in choosing which clause you will use to specify a filter expression. Because logically it makes no difference where you place your filters, and typically there's also no performance difference, your guideline should be natural and intuitive writing. Write in a way that feels more natural to you and to the programmers who need to maintain your code. For example, to me a filter that matches attributes between the tables should appear in the ON clause, while a filter on an attribute from only one table should appear in the WHERE clause. I'll use the following query to return orders placed by U.S. customers: USE Northwind; SELECT C.CustomerID, CompanyName, OrderID FROM dbo.Customers AS C JOIN dbo.Orders AS O ON C.CustomerID = O.CustomerID WHERE Country = 'USA';
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Using the ANSI SQL:1989 syntax, you have no choice but to specify all filter expressions in the WHERE clause: SELECT C.CustomerID, CompanyName, OrderID FROM dbo.Customers AS C, dbo.Orders AS O WHERE C.CustomerID = O.CustomerID AND Country = 'USA';
Remember that the discussion here is about inner joins; with outer joins, there are logical differences between specifying a filter expression in the ON clause and specifying it in the WHERE clause. I mentioned earlier that I like using the ANSI SQL:1989 syntax for cross joins. However, with inner joins, my recommendation is different. The reason for this is that there's a risk in using the ANSI SQL:1989 syntax. If you forget to specify the join condition, unintentionally you get a cross join, as demonstrated in the following code: SELECT C.CustomerID, CompanyName, OrderID FROM dbo.Customers AS C, dbo.Orders AS O;
In SQL Server Management Studio (SSMS), the query plan for a cross join will include a join operator marked with a yellow warning symbol, and the pop-up details will say "No Join Predicate" in the Warnings section. This warning is designed to alert you that you might have forgotten to specify a join predicate. However, if you explicitly specify INNER JOIN when you write an inner join query, an ON clause is required. If you forget to specify any join condition, the parser traps the error and the query is not run: SELECT C.CustomerID, CompanyName, OrderID FROM dbo.Customers AS C JOIN dbo.Orders AS O; Msg 102, Level 15, State 1, Line 2 Incorrect syntax near ';'.
The parser finds a semicolon after dbo.Orders AS O,even though it expects something else (an ON clause or other options), so it generates an error saying that there's incorrect syntax near ';'.
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