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Note that you can call Close() on a closed connection, and no exception will be thrown. So, your message would have been displayed if the connection had been closed earlier, or even if it had never been opened. See Figure 5-2, where the connection failed but the close message was still displayed. In one typical case, multiple calls to both Open() and Close() make sense. ADO.NET supports disconnected processing of data, even when the connection to the data provider has been closed. The pattern looks like this:

The SELECT statement in the derived table contains an expression that returns the year of the OrderDate, the OrderYear, and an expression that returns the month of the OrderDate, OrderMonth It also contains the TotalDue column The query will group the results by OrderYear The OrderMonth column is the pivoted column The query will sum up the TotalDue values The derived table contains only the columns and expressions needed by the pivoted query..

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In the Components tray, open the smart tag for the ProductSubcategoryTableAdapter and click the Add Query link. After the click, the Search Criteria Builder dialog box opens. You must specify the syntax for a new query, and you must assign the new query a name (or you can use a previously specified query). The query name specifies a FillBy method that you can invoke with code for more customized solutions. However, the basic approach described here requires no code. In the current example, assign the name FillByCatID as the name of the query. The syntax for the query within the Search Criteria Builder dialog box appears in Figure 13-21. Figure 13-22 shows Form8 after it initially opens and after a value of 1 is entered as an @CatID parameter. The top view of Form8 shows the form when it opens. Notice that the BindingNavigator permits navigation to any one of 37 rows in the DataGridView control. This view of the data returned by the ProductSubcategoryTableAdapter includes no parameters. However, you can see the control for entering a parameter value above the BindingNavigator control. A TextBox is available for specifying an @CatID parameter value, and the text to the right of the TextBox can be clicked to invoke a parameter query that populates the DataGridView control with the results of the query. After entering a value of 1 for the parameter and clicking FillByCatID1, the DataGridView control displays just three rows. Each of these rows has a ProductCategoryID column value of 1. You can change the contents of the DataGridView control to display any other subset of rows based on ProductCategoryID column values by inputting a new value for @CatID and clicking FillByCatID1.

Try conn.Open() ' Open connection ' ' Online processing (e.g., queries) here ' conn.Close() ' Close connection

Figure 13-21. You can create a parameter query for a DataGridView control as simply as naming the query and adding a WHERE clause in a dialog box.

' ' Offline processing here ' conn.Open() ' Reopen connection ' ' Online processing (e.g., INSERT/UPDATE/DELETE) here ' conn.Close() ' Reclose connection Finally ' Close connection conn.Close() End Try

Figure 13-22. Creating basic parameter queries for a DataGridView control is easy and powerful with the TableAdapter object.

Figure 10-11. The results of using PIVOT The PIVOT function specifies the aggregate expression SUM(TotalDue). The pivoted column is OrderMonth. The IN expression contains the numbers 1 6, each surrounded by brackets. The IN expression lists the values for OrderMonth that you want to show up in the final results. These values are also the column names. Since columns starting with numbers are not valid column names, the brackets surround the numbers. You could also quote these numbers. The IN expression has two purposes: to provide the column names and to filter the results. The outer SELECT list contains OrderYear and the numbers 1 6 surrounded with brackets. These must be the same values found in the IN expression. Because you want the month abbreviations instead of numbers as the column names, the query uses aliases. Notice that the SELECT list does not contain the TotalDue column. Finally, the ORDER BY clause specifies that the results will sort by OrderYear. The results of query 2 are identical to the results from the pivoted results using the CASE technique in the previous section. This query uses the ROUND and ISNULL functions to replace NULL with zero and round the results.

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