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Updating the Database
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Most applications that work with databases need to update data in the original tables sooner or later. With ADO.NET, you have two choices when it s time to update a database:
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ADO.NET in Disconnected Mode
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You can use ADO.NET Command objects with appropriate INSERT, DELETE, and UPDATE SQL statements. This is what you usually do when you work in con nected mode and read data by means of a DataReader object. You can use the Update method of the DataAdapter object to send changed rows in a DataSet to a database. In this case, you usually use the same DataAdapter object that you created to read data into the DataSet, even though this isn t a requirement. (For example, you might have filled the DataSet manually via code without using a DataAdapter object.)
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The real issue when working in disconnected mode is that you have to detect and resolve update conflicts. You have a conflict when another user has modified or deleted the same record that you want to update or delete, or has inserted a new record that has the same primary key as a record that you have inserted. How your application reacts to a conflict depends on the application s own logic for example, you might follow the simple strategy by which the first update wins and subsequent updates are ignored; or you might decide that the last update wins. I ll explain these conflict-resolution strategies later in this chapter; for now, let s focus on the basics of update operations under the simplistic assumption that there are no update conflicts, an assumption that s realistic only when you re working with single-user applications.
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The code in the following sections modifies the Biblio.mdb demo database or the Pubs database. Before running this code, you might want to make a copy of the database so that you can restore it later. Also notice that you might need to restore the database before running the same sample again for example, if you want to compare the outcomes of differ ent update strategies.
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Getting Familiar with Update Concepts
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You can update data in a DataSet by means of the DataAdapter s Update method, which takes one of the following sets of arguments: a reference to a DataTable object; a DataSet and the name of a DataTable it contains; an array of DataRow objects. You usually use one of the first two syntax forms, and pass a DataRow array only when you want more control on the order in which changed rows are submitted to the database. In all cases, the Update method returns the number of rows that have been successfully updated. The key to performing batch updates with ADO.NET is a group of three properties of the DataAdapter object: InsertCommand, UpdateCommand, and DeleteCommand. Here s how the update mechanism works. When an Update command is issued, the DataAdapter checks the RowState property of each row specified as a source for the update operation. If the state is Added, the DataAdapter issues the SQL command specified in the InsertCommand property. If the state
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Part V:
Database Applications
is Modified, the DataAdapter uses the SQL command in the UpdateCommand property. If the state is Deleted, the command in the DeleteCommand property is used instead. (See Figure 22-6.)
DataSet DataRow DataRow DataRow Deleted DataAdapter Modified UpdateCommand DeleteCommand Database
Figure 22-6
How the DataAdapter s Update method works
The InsertCommand, UpdateCommand, and DeleteCommand properties must be assigned actual ADO.NET Command objects with parameters. You can create these commands yourself or generate them more easily by using an auxiliary CommandBuilder object. The main drawbacks of the latter technique are that the auxiliary Com mandBuilder object must execute the SELECT command to retrieve the metadata, so it requires an additional round trip to the server and adds overhead to your application. Because of its simplicity, however, I ll explain the technique based on the CommandBuilder object first. (There is also a third technique based on Visual Studio .NET code generation features, as you ll see in 23.) Each .NET data provider comes with its own CommandBuilder class, so you ll work with either the OleDbCommandBuilder, the SqlCommandBuilder, the OdbcCommandBuilder, or the OracleCommandBuilder object. The following code snippet creates a DataAdapter from a simple SELECT statement and then uses the CommandBuilder object to generate the three xxxCommand properties:
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