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Interactive data displays allow the data on a form to change in response to user input. For example, a user might want to look up information about an item in a database, such as a product or a person. Nearly all the samples to this point in the chapter focus on returning a fixed result set, such as a set of column values from the ProductCategory database table in the AdventureWorks database. This section will demonstrate the application of built-in and customized techniques for allowing users to change the results that they display on a form. It is one thing to see results change based on user input, but it is also important to be able to change the results in a database based on user input. The default way of committing changes to a database with graphical approaches, such as those covered in this chapter, is through optimistic concurrency. This approach is preferred because it increases the number of users that a database can serve in comparison to another popular approach called pessimistic locking. However, one disadvantage of optimistic concurrency is that someone else can change data before you commit your change to a database. In this case, your change must be rejected because the database cannot find the base record that you want to modify (after all, someone else modified it already). This chapter demonstrates an easy technique for handling errors resulting from database concurrency, which happen when two different users try to change the same row in a database table or view.

The classic way to make results dynamic at runtime is with a parameter query A parameter query is a regular SELECT statement with a WHERE clause that has one or more key-value pairs The key is a column name in the data source specified by the FROM clause for the SELECT statement The value is a parameter or a criterion The WHERE clause contains an operator, such as an equal sign (=), between each key-value pair that returns a value of True or False If your parameter query has more than one key-value pair, then you must have operators between pairs so that the overall parameter query returns a value of True or False The parameter query returns a subset of rows from the original query for parameter expression values equal to True In a sense, a parameter query looks up information for rows that match its search criteria.

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Finally ' Close connection conn.Close() Console.WriteLine("Connection closed.")

Pivoting Results with PIVOT USE AdventureWorks2008; GO --1 SELECT OrderYear, [1] AS Jan, [2] AS Feb, [3] AS Mar, [4] AS Apr, [5] AS May, [6] AS Jun FROM (SELECT YEAR(OrderDate) AS OrderYear, TotalDue, MONTH(OrderDate) AS OrderMonth FROM SalesSalesOrderHeader) AS MonthData PIVOT ( SUM(TotalDue) FOR OrderMonth IN ([1],[2],[3],[4],[5],[6]) ) AS PivotData ORDER BY OrderYear; --2 SELECT OrderYear, ROUND(ISNULL([1],0),0) AS Jan, ROUND(ISNULL([2],0),0) AS Feb, ROUND(ISNULL([3],0),0) AS Mar, ROUND(ISNULL([4],0),0) AS Apr, ROUND(ISNULL([5],0),0) AS May, ROUND(ISNULL([6],0),0) AS Jun FROM (SELECT YEAR(OrderDate) AS OrderYear, TotalDue, MONTH(OrderDate) AS OrderMonth FROM SalesSalesOrderHeader) AS MonthData PIVOT ( SUM(TotalDue) FOR OrderMonth IN ([1],[2],[3],[4],[5],[6]) ) AS PivotData ORDER BY OrderYear; Figure 10-11 shows the results First take a look at the derived table aliased as MonthData in query 1.

Parameter queries are so popular that Microsoft has traditionally made them very easy to use in its database applications, and the tradition continues with TableAdapters for a DataGridView control Recall that a DataGridView control relies on a TableAdapter The Windows Form Designer automatically adds the TableAdapter for the DataGridView control to a Components tray below the form containing the DataGridView control You can add a new query to the TableAdapter from the smart tag window for the TableAdapter This query should contain a WHERE clause for a parameter query After completing the query, the Windows Form Designer adds a control to the form that allows users to specify parameter values and run the parameter query to specify a subset of values from the original query to populate the DataGridView control.

s Note Establishing connections (database sessions) is relatively expensive. They use resources on both the client and the server. Although connections might eventually get closed, through garbage collection or by timing out, leaving one open when it s no longer needed is a bad practice. Too many open connections can slow a server down or prevent new connections from being made.

You can start to create your first parameter query graphically by dragging the ProductSubCategory node from the CatSubCatAdventureWorks_DataDataSet data source in the Data Sources window for the WinCh13 project This adds a DataGridView control and a corresponding BindingNavigator control to a new form, such as Form8 Edit the columns for the DataGridView control to drop the rowguid and ModifiedDate columns The Load event procedure for Form8 includes the following assignment statement for dynamically resizing the ProductSubcategoryID column in the DataGridView control so that the column is initially wide enough to display its heading without cropping MeProductSubcategoryDataGridView _ Columns("ProductSubcategoryID")MinimumWidth = 115.

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