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try { con.Open(); adapter.Fill(dsPubs, "Authors"); // This command is still linked to the data adapter. cmd.CommandText = "SELECT au_id, title_id FROM TitleAuthor"; adapter.Fill(dsPubs, "TitleAuthor"); // This command is still linked to the data adapter. cmd.CommandText = "SELECT title_id, title FROM Titles"; adapter.Fill(dsPubs, "Titles"); } catch (Exception err) { lblList.Text = "Error reading list of names. "; lblList.Text += err.Message; } finally { con.Close(); }

The easiest way to interact with a database is to use direct data access. When you use direct data access, you re in charge of building a SQL command (like the ones you considered earlier in this chapter) and executing it. You use commands to query, insert, update, and delete information. When you query data with direct data access, you don t keep a copy of the information in memory. Instead, you work with it for a brief period of time while the database connection is open, and then close the connection as soon as possible. This is different than disconnected data access, where you keep a copy of the data in the DataSet object so you can work with it after the database connection has been closed. The direct data model is well suited to ASP.NET web pages, which don t need to keep a copy of their data in memory for long periods of time. Remember, an ASP.NET web page is loaded when the page is requested and shut down as soon as the response is returned to the user. That means a page typically has a lifetime of only a few seconds (if that).

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Now that all the information is in the DataSet, you can create two DataRelation objects to make it easier to navigate through the linked information. In this case, these DataRelation objects match the foreign key restrictions that are defined in the database.

tasks, they just might use this technique to optimize performance. For example, you could get the product catalog from a database once, and keep that data in memory on the web server so you can reuse it when someone else requests the same page. This technique is called caching, and you ll learn to use it in 24.

to a randomized number. This is to avoid HTTP caching of the Ajax URL. You could also achieve the same result by sending the correct cache headers with the response to prevent caching of the open URL.

Note A foreign key is a constraint that you can set up in your database to link one table to another. For

To query information with simple data access, follow these steps: 1. Create Connection, Command, and DataReader objects. 2. Use the DataReader to retrieve information from the database, and display it in a control on a web form. 3. Close your connection. 4. Send the page to the user. At this point, the information your user sees and the information in the database no longer have any connection, and all the ADO.NET objects have been destroyed. To add or update information, follow these steps: 1. Create new Connection and Command objects. 2. Execute the Command (with the appropriate SQL statement). This chapter demonstrates both of these approaches. Figure 15-9 shows a high-level look at how the ADO.NET objects interact to make direct data access work.

example, the TitleAuthor table is linked to the Titles and the Authors tables by two foreign keys. The title_id field in the TitleAuthor table has a foreign key that binds it to the title_id field in the Titles table. Similarly, the au_id field in the TitleAuthor table has a foreign key that binds it to the au_id field in the Authors table. Once these links are established, certain rules come into play. For example, you can t create a TitleAuthor record that specifies author or title records that don t exist.

Figure 15-9. Direct data access with ADO.NET Before continuing, make sure you import the ADO.NET namespaces. In this chapter, we assume you re using the SQL Server provider, in which case you need these two namespace imports: Imports System.Data Imports System.Data.SqlClient

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