Using a SqlCeDataAdapter Object to Update a Database in Visual Studio .NET

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6382 Using a SqlCeDataAdapter Object to Update a Database
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Remember that the SqlCeDataAdapter object lies between the DataSet object and the underlying database The SqlCeDataAdapter object is used to pull the data from the database and place it into the data table, and it can be used to push any changes made to the data in the data table back to the database Just as a SqlCeDataAdapter object has a SelectCommand property, it also has InsertCommand, UpdateCommand, and DeleteCommand properties, each holding a SqlCeCommand object These commands are used to push changes made to the data tables back to the database They are called by the SqlCeDataAdapter object s Update method You can call them directly, but normally you let the Update method do it for you Coming up with the parameterized SQL statements for these command objects may be very simple or quite difficult It all depends on the complexity of the SELECT statement in the SelectCommand property The SqlCeDataAdapter object can build the InsertCommand, UpdateCommand, and DeleteCommand command objects for you if you give it some help (which is provided by the SqlCeCommandBuilder class) and if all of the following conditions are true
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The SelectCommand property s SELECT statement accesses just one
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database table The table being accessed has a primary key or unique constraint The columns that make up that constraint are in the SELECT clause of the SELECT statement Each data element in the output of the SELECT statement can be traced back to a single data element in the database (no literals, aggregates, or functions) If your SELECT statement meets these criteria, and if you have created a SqlCeCommandBuilder object and associated it with the SqlCeDataAdapter object prior to calling the SqlCeDataAdapter object s Update method, the
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63 Microsoft SQL Server CE SqlCeCommandBuilder object builds the Insert, Update, and Delete com-
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mand objects for you To illustrate this, we add a simple form to our project for updating category information, as shown in Figure 614
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Figure 614: The Category Modification Form
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Listing 69 shows the code for this form Most of it is familiar, consisting of creating and loading the data set and binding to the controls New concepts in the code are the creation of the SqlCeCommandBuilder object in the Load event handler and the update-specific code in the Closing event handler
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Listing 69: The Category Modification Form Class
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using using using using using using using using System; SystemData; SystemDataCommon; SystemDataSqlServerCe; SystemDrawing; SystemCollections; SystemComponentModel; SystemWindowsForms;
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namespace CreateDatabase { /// <summary> /// Summary description for FormUpdate /// </summary> public class FormUpdate : SystemWindowsFormsForm { internal SystemWindowsFormsPanel panelCategory;
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internal internal internal internal internal SystemWindowsFormsComboBox comboCategoryIDs; SystemWindowsFormsTextBox textCategoryName; SystemWindowsFormsLabel lblCategoryName; SystemWindowsFormsTextBox textCategoryID; SystemWindowsFormsLabel lblCategoryID;
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public FormUpdate() { InitializeComponent(); } protected override void Dispose( bool disposing ) { baseDispose( disposing ); } #region Windows Form Designer generated code // Connection string private string strConn = "Data Source=" + @"My Documents\ourProduceCosdf"; // The private private private data set, adapter, table DataSet dsetDB; SqlCeDataAdapter daptCategories; DataTable dtabCategories;
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private bool boolLoading = true;
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private void FormUpdate_Load(object sender, EventArgs e) // Present a close box { thisMinimizeBox = false; // Create the data set dsetDB = new DataSet("Produce"); // Create the data adapter daptCategories = new SqlCeDataAdapter("SELECT CategoryID, CategoryName " + " FROM Categories", strConn); // Create the command builder for the adapter SqlCeCommandBuilder cbldCategories = new SqlCeCommandBuilder(daptCategories); // // Create and fill the data table, and save a reference to it
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63 Microsoft SQL Server CE
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daptCategoriesFill(dsetDB, "Categories"); dtabCategories = dsetDBTables["Categories"]; // Bind the combo box with the category names comboCategoryIDsDataSource = dtabCategories; comboCategoryIDsDisplayMember = "CategoryName"; comboCategoryIDsValueMember = "CategoryID"; comboCategoryIDsSelectedIndex = 0; // Load labels with data table column names lblCategoryIDText = "CategoryID"; lblCategoryNameText = "CategoryName"; // Bind the data table's columns to the text boxes textCategoryIDDataBindingsAdd ("Text", dtabCategories, "CategoryID"); textCategoryNameDataBindingsAdd ("Text", dtabCategories, "CategoryName"); // Give the panel some tone panelCategoryBackColor = ColorBeige; // Loading is finished boolLoading = false; comboCategoryIDsSelectedIndex = 0; } private void textCategoryName_Validated(object sender, EventArgs e) { // Force the current modification to complete thisBindingContext[dtabCategories]EndCurrentEdit(); } private void FormUpdate_Closing(object sender, CancelEventArgs e) { // Force the current modification to complete thisBindingContext[dtabCategories]EndCurrentEdit(); // Push data set changes back to the database daptCategoriesUpdate(dsetDB, "Categories"); } } }
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Regardless of whether you are letting the SqlCeCommandBuilder object build the adapter s three modification commands or you are building them programmatically yourself, calling the SqlCeDataAdapter object s
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flict detection prevents an application from modifying data that has been modified by some other application since the first application read the data This ensures users that they are updating data whose values are still the values being presented to the user In a multiuser environment, conflict detection is a very beneficial guardian; in the single-user world, it is a lot of overhead If the user runs two applications to update the same data at the same time, the SqlCeDataAdapter object quite rightly lets the user trash his or her own updates if the user chooses to do so Even though you use an adapter to bring data into a data set, using it to push changes back to the database is always optional You can always write your own DML statements and place them into the CommandText properties of the adapter s command objects As we mentioned, if the SELECT statement is sufficiently complex, the SqlCeCommandBuilder object cannot generate the Insert, Update, and Delete statements; you have to write them yourself You might want to write those statements yourself for additional control or application documentation reasons Your application can always create a connection and a command object and use the command object s ExecuteNonQuery method to submit an update to a database And for reasons we will explain in the upcoming Microsoft SQL Server section, you are more likely to provide your own updating when connected to SQL Server rather than to SQL Server CE But first we examine one last aspect of SQL Server CE programming: obtaining information about a database s schema
639 Querying Schema Information In an environment where data is being collected from a variety of sources and stored in your data tables, knowing the schema of that data can be