Multitable Data Sets in Visual Studio .NET

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Data sets can hold more than one table at a time When creating data sets that contain multiple tables, you will want to use one data adapter for each table loaded In addition, you must be careful when filling a data set using more than one adapter If you call the data adapter's Fill method on a data set multiple times, you'll end up appending data into a single table, so you need to be specific about what table you're trying to fill: // Configure the connection SqlConnection conn = new SqlConnection(@"");
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// Create the adapters SqlDataAdapter customersAdapter = new SqlDataAdapter(); SqlDataAdapter ordersAdapter = new SqlDataAdapter(); // Create a data set DataSet dataset = new DataSet(); void MultiTableForm_Load(object sender, EventArgs e) { // Create the Customer adapter from the connection customersAdapterSelectCommand = connCreateCommand(); customersAdapterSelectCommandCommandText = "select * from customers"; // Fill the data set with the Customers table customersAdapterFill(dataset, "Customers"); // Create the Orders adapter from the connection ordersAdapterSelectCommand = connCreateCommand(); ordersAdapterSelectCommandCommandText = "select * from orders"; // Fill the data set with the Orders table ordersAdapterFill(dataset, "Orders"); // Need one command builder for each adapter // in anticipation of eventually committing changes new SqlCommandBuilder(customersAdapter); new SqlCommandBuilder(ordersAdapter); // Populate list boxes PopulateListBoxes(); } This code fills a data set with data from two different data adapters, one for each table When you call the Fill method of the data adapter, you must specify which table to fill with the data from the adapter If you fail to do this, you will get one data table (called "Table") with data from both Fill methods You could have used a single data adapter to fill both tables, but because the command builders use the SelectCommand to determine how to update the data provider, it is good form to have a one-to-one relationship between tables in the data set and data adapters Notice that when the data adapters are created, one command builder is created for each of them in anticipation of committing changes for each table With more than one table, the code to commit changes needs to be updated: void commitChangesMenuItem_Click(object sender, EventArgs e) { // Commit customer changes back to the data provider try { customersAdapterUpdate(dataset, "Customers"); } catch( SqlException ex ) { MessageBoxShow(exMessage, "Error(s) Committing Customer Changes"); } // Commit order changes back to the data provider try { ordersAdapterUpdate(dataset, "Orders"); } catch( SqlException ex ) { MessageBoxShow(exMessage, "Error(s) Committing Order Changes");
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} // Update list boxes PopulateListBoxes(); } This code commits changes on each table by calling the Update method of the particular data adapter while specifying the table to update Make sure not to get the adapter mixed up with the name of the table, or things won't go so well
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If you'd like to catch problems with the data as it's added by the user instead of waiting until the data is sent back to the data provider, you can establish constraints A constraint limits the kind of data that can be added to each column The SystemData namespace comes with two constraints: the foreign key constraint and the unique value constraint, which are represented by the ForeignKeyConstraint and the UniqueConstraint classes, respectively For example, to make sure that no two rows have the same value in a column, you can add a unique constraint to the table's list of constraints: // Add a constraint DataTable customers = datasetTables["Customers"]; UniqueConstraint constraint = new UniqueConstraint(customersColumns["CustomerID"]); customersConstraintsAdd(constraint); With the constraint in place, if a row is added to the table that violates the constraint, a runtime exception will be thrown immediately, without a round-trip to the data provider Unique constraints set up a constraint between one or more columns in a single table, whereas foreign key constraints set up an existence requirement between columns in multiple tables Foreign key constraints are set up automatically whenever a relation is established
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