The output from this program is as follows: numberOfRows = 3 Reading from the Products DataTable ProductID = 1 ProductName = Chai UnitPrice = 18 ProductID = 2 ProductName = Chang UnitPrice = 19 Reading from the Customers DataTable CustomerID = ALFKI CompanyName = Alfreds Futterkiste Changing the CommandText Property of the SelectCommand You can also populate a DataSet with multiple DataTable objects by changing the CommandText property of the SelectCommand for your DataAdapter object before each call to the Fill() method. First, the following code populates a DataSet with a DataTable containing two rows from the Products table: SqlCommand mySqlCommand = mySqlConnection.CreateCommand(); mySqlCommand.CommandText = "SELECT TOP 2 ProductID, ProductName, UnitPrice " + "FROM Products " + "ORDER BY ProductID"; SqlDataAdapter mySqlDataAdapter = new SqlDataAdapter(); mySqlDataAdapter.SelectCommand = mySqlCommand; DataSet myDataSet = new DataSet(); mySqlConnection.Open(); int numberOfRows = mySqlDataAdapter.Fill(myDataSet, "Products"); The myDataSet object now contains a DataTable named Products. Next, the CommandText property for the SelectCommand of mySqlDataAdapter is changed to a SELECT statement that retrieves rows from the Customers table, and the Fill() method is called again: mySqlDataAdapter.SelectCommand.CommandText = "SELECT CustomerID, CompanyName " + "FROM Customers " + "WHERE CustomerID = 'ALFKI'"; numberOfRows = mySqlDataAdapter.Fill(myDataSet, "Customers"); mySqlConnection.Close(); The myDataSet object now contains an additional DataTable named Customers. Listing 10.6 shows a program that uses the code examples shown in this section. Listing 10.6: MULTIPLEDATATABLES2.CS /* MutlipleDataTables2.cs illustrates how to populate a DataSet object with multiple DataTable objects by changing the CommandText property of a DataAdapter object's SelectCommand */ using System; using System.Data; using System.Data.SqlClient; class MultipleDataTables2 { public static void Main() { SqlConnection mySqlConnection = new SqlConnection( "server=localhost;database=Northwind;uid=sa;pwd=sa" ); SqlCommand mySqlCommand = mySqlConnection.CreateCommand(); mySqlCommand.CommandText = "SELECT TOP 2 ProductID, ProductName, UnitPrice " + "FROM Products " + "ORDER BY ProductID"; SqlDataAdapter mySqlDataAdapter = new SqlDataAdapter(); mySqlDataAdapter.SelectCommand = mySqlCommand; DataSet myDataSet = new DataSet(); mySqlConnection.Open(); int numberOfRows = mySqlDataAdapter.Fill(myDataSet, "Products"); Console.WriteLine("numberOfRows = " + numberOfRows); // change the CommandText property of the SelectCommand mySqlDataAdapter.SelectCommand.CommandText = "SELECT CustomerID, CompanyName " + "FROM Customers " + "WHERE CustomerID = 'ALFKI'"; numberOfRows = mySqlDataAdapter.Fill(myDataSet, "Customers"); Console.WriteLine("numberOfRows = " + numberOfRows); mySqlConnection.Close(); foreach (DataTable myDataTable in myDataSet.Tables) { Console.WriteLine("\nReading from the " + myDataTable.TableName + "DataTable"); foreach (DataRow myDataRow in myDataTable.Rows) { foreach (DataColumn myDataColumn in myDataTable.Columns) { Console.WriteLine(myDataColumn + "= " + myDataRow[myDataColumn]); } } } } } The output from this program is as follows: numberOfRows = 2 numberOfRows = 1 Reading from the Products DataTable ProductID = 1 ProductName = Chai UnitPrice = 18 ProductID = 2 ProductName = Chang UnitPrice = 19 Reading from the Customers DataTable CustomerID = ALFKI CompanyName = Alfreds Futterkiste Using Multiple DataAdapter Objects to Populate the Same DataSet Object You can also populate the same DataSet with multiple DataTable objects using different DataAdapter objects. For example, assume you already have a DataSet named myDataSet that was populated using a SqlDataAdapter named mySqlDataAdapter, and that myDataSet currently contains a DataTable named Products. The following example creates another SqlDataAdapter and uses it to populate myDataSet with another DataTable named Customers: SqlDataAdapter mySqlDataAdapter2 = new SqlDataAdapter(); mySqlDataAdapter2.SelectCommand = mySqlCommand; mySqlDataAdapter2.SelectCommand.CommandText = "SELECT CustomerID, CompanyName " + "FROM Customers " + "WHERE CustomerID = 'ALFKI'"; numberOfRows = mySqlDataAdapter2.Fill(myDataSet, "Customers"); Listing 10.7 shows a program that uses the code examples shown in this section. Listing 10.7: MULTIPLEDATATABLES3.CS /* MutlipleDataTables3.cs illustrates how to populate a DataSet object with multiple DataTable objects using multiple DataAdapter objects to populate the same DataSet object */ using System; using System.Data; using System.Data.SqlClient; class MultipleDataTables3 { public static void Main() { SqlConnection mySqlConnection = new SqlConnection( "server=localhost;database=Northwind;uid=sa;pwd=sa" ); SqlCommand mySqlCommand = mySqlConnection.CreateCommand(); mySqlCommand.CommandText = "SELECT TOP 2 ProductID, ProductName, UnitPrice " + "FROM Products " + "ORDER BY ProductID"; SqlDataAdapter mySqlDataAdapter1 = new SqlDataAdapter(); mySqlDataAdapter1.SelectCommand = mySqlCommand; DataSet myDataSet = new DataSet(); mySqlConnection.Open(); int numberOfRows = mySqlDataAdapter1.Fill(myDataSet, "Products"); Console.WriteLine("numberOfRows = " + numberOfRows); // create another DataAdapter object SqlDataAdapter mySqlDataAdapter2 = new SqlDataAdapter(); mySqlDataAdapter2.SelectCommand = mySqlCommand; mySqlDataAdapter2.SelectCommand.CommandText = "SELECT CustomerID, CompanyName " + "FROM Customers " + "WHERE CustomerID = 'ALFKI'"; numberOfRows = mySqlDataAdapter2.Fill(myDataSet, "Customers"); Console.WriteLine("numberOfRows = " + numberOfRows); mySqlConnection.Close(); foreach (DataTable myDataTable in myDataSet.Tables) { Console.WriteLine("\nReading from the " + myDataTable.TableName + "DataTable"); foreach (DataRow myDataRow in myDataTable.Rows) { foreach (DataColumn myDataColumn in myDataTable.Columns) { Console.WriteLine(myDataColumn + "= " + myDataRow[myDataColumn]); } } } } } The output from this program is as follows: numberOfRows = 2 numberOfRows = 1 Reading from the Products DataTable ProductID = 1 ProductName = Chai UnitPrice = 18 ProductID = 2 ProductName = Chang UnitPrice = 19 Reading from the Customers DataTable CustomerID = ALFKI CompanyName = Alfreds Futterkiste Merging DataRow, DataSet, and DataTable Objects into Another DataSet In this section, you'll learn how to use the Merge() method to merge DataRow, DataSet, and DataTable objects into another DataSet. You might want to do this when you have multiple sources of data; for example, you might get data from many regional offices that is sent to headquarters, and you need to merge all that data into one DataSet. The Merge() method is overloaded as follows: void Merge(DataRow[] myDataRows) void Merge(DataSet myDataSet) void Merge(DataTable myDataTable) void Merge(DataSet myDataSet, bool preserveChanges) void Merge(DataRow[] myDataRows, bool preserveChanges, MissingSchemaAction myMissingSchemaAction) void Merge(DataSet myDataSet, bool preserveChanges, MissingSchemaAction myMissingSchemaAction) void Merge(DataTable myDataTable, bool preserveChanges, MissingSchemaAction myMissingSchemaAction) where • PreserveChanges specifies whether changes in the current DataSet (the DataSet with the Merge() method that is called) are to be kept. • MyMissingSchemaAction specifies the action to take when the current DataSet doesn't have the same tables or columns as the DataRow, DataSet, or DataTable being merged into that DataSet. You set myMissingSchemaAction to one of the constants defined in the System.Data.MissingSchemaAction enumeration. Table 10.7 shows the constants defined in the MissingSchemaAction enumeration. Table 10.7: MissingSchemaAction ENUMERATION MEMBERS CONSTANT DESCRIPTION Add The column or table is added to the current DataSet. Add is the default. Table 10.7: MissingSchemaAction ENUMERATION MEMBERS CONSTANT DESCRIPTION AddWithKey The column and primary key information is added to the current DataSet. Error A SystemException is thrown. Ignore The column or table is ignored and not read. Listing 10.8 illustrates the use of the Merge() method. Listing 10.8: MERGE.CS /* Merge.cs illustrates how to use the Merge() method */ using System; using System.Data; using System.Data.SqlClient; class Merge { public static void Main() { SqlConnection mySqlConnection = new SqlConnection( "server=localhost;database=Northwind;uid=sa;pwd=sa" ); SqlCommand mySqlCommand = mySqlConnection.CreateCommand(); // populate myDataSet with three rows from the Customers table mySqlCommand.CommandText = "SELECT CustomerID, CompanyName, ContactName, Address " + "FROM Customers " + "WHERE CustomerID IN ('ALFKI', 'ANATR', 'ANTON')"; SqlDataAdapter mySqlDataAdapter = new SqlDataAdapter(); mySqlDataAdapter.SelectCommand = mySqlCommand; DataSet myDataSet = new DataSet(); mySqlConnection.Open(); mySqlDataAdapter.Fill(myDataSet, "Customers"); // populate myDataSet2 with two rows from the Customers table mySqlCommand.CommandText = "SELECT CustomerID, CompanyName, ContactName, Address " + "FROM Customers " + "WHERE CustomerID IN ('AROUT', 'BERGS')"; DataSet myDataSet2 = new DataSet(); mySqlDataAdapter.Fill(myDataSet2, "Customers2"); // populate myDataSet3 with five rows from the Products table mySqlCommand.CommandText = "SELECT TOP 5 ProductID, ProductName, UnitPrice " + "FROM Products " + "ORDER BY ProductID"; DataSet myDataSet3 = new DataSet(); mySqlDataAdapter.Fill(myDataSet3, "Products"); mySqlConnection.Close(); // merge myDataSet2 into myDataSet myDataSet.Merge(myDataSet2); // merge myDataSet3 into myDataSet myDataSet.Merge(myDataSet3, true, MissingSchemaAction.Add); // display the rows in myDataSet foreach (DataTable myDataTable in myDataSet.Tables) { Console.WriteLine("\nReading from the " + myDataTable + "DataTable"); foreach (DataRow myDataRow in myDataTable.Rows) { foreach (DataColumn myDataColumn in myDataTable.Columns) { Console.WriteLine(myDataColumn + "= " + myDataRow[myDataColumn]); } } } } } The output from this program is as follows: Reading from the Customers DataTable CustomerID = ALFKI CompanyName = Alfreds Futterkiste ContactName = Maria Anders Address = Obere Str. 57 CustomerID = ANATR CompanyName = Ana Trujillo3 Emparedados y helados ContactName = Ana Trujillo Address = Avda. de la Constitución 2222 CustomerID = ANTON CompanyName = Antonio Moreno Taquería ContactName = Antonio Moreno Address = Mataderos 2312 Reading from the Customers2 DataTable CustomerID = AROUT CompanyName = Around the Horn ContactName = Thomas Hardy Address = 120 Hanover Sq. CustomerID = BERGS CompanyName = Berglunds snabbköp ContactName = Christina Berglund Address = Berguvsvägen 8 Reading from the Products DataTable ProductID = 1 ProductName = Chai UnitPrice = 18 ProductID = 2 ProductName = Chang UnitPrice = 19 ProductID = 3 ProductName = Aniseed Syrup UnitPrice = 10 ProductID = 4 ProductName = Chef Anton's Cajun Seasoning UnitPrice = 22 ProductID = 5 ProductName = Chef Anton's Gumbo Mix UnitPrice = 21.35 . merge myDataSet2 into myDataSet myDataSet.Merge(myDataSet2); // merge myDataSet3 into myDataSet myDataSet.Merge(myDataSet3, true, MissingSchemaAction.Add);. DataSet myDataSet2 = new DataSet( ); mySqlDataAdapter.Fill(myDataSet2, "Customers2"); // populate myDataSet3 with five rows from the