Modifying Rows in a DataTable phần 1

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Modifying Rows in a DataTable phần 1

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Modifying Rows in a DataTable In this section, you'll see the steps required to add, modify, and remove DataRow objects from a DataTable and then push those changes to the database

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  1. Modifying Rows in a DataTable In this section, you'll see the steps required to add, modify, and remove DataRow objects from a DataTable and then push those changes to the database. The examples in this section show how to add, modify, and delete rows in the Customers database table. Note You'll find a complete program named AddModifyAndRemoveDataRows.cs in the ch11 directory that illustrates the use of the methods shown in this section. This program listing is omitted from this book for brevity. Setting up a DataAdapter to Push Changes to the Database In Chapter 10, you saw that before you call the Fill() method of your DataAdapter to read rows from the database, you first need to set the SelectCommand property of your DataAdapter. For example: SqlCommand mySelectCommand = mySqlConnection.CreateCommand(); mySelectCommand.CommandText = "SELECT CustomerID, CompanyName, Address " + "FROM Customers " + "ORDER BY CustomerID"; SqlDataAdapter mySqlDataAdapter = new SqlDataAdapter(); mySqlDataAdapter.SelectCommand = mySelectCommand; The SELECT statement is then run when you call the mySqlDataAdapter object's Fill() method to retrieve rows from the Customers table into a DataSet. Similarly, before you can push changes to the database, you must first set up your DataAdapter with Command objects containing appropriate SQL INSERT, UPDATE, and DELETE statements. You store these Command objects in your DataAdapter object's InsertCommand, UpdateCommand, and DeleteCommand properties. You push changes from your DataSet to the database using the Update() method of your DataAdapter. When you add, modify, or remove DataRow objects from your DataSet and then call the Update() method of your DataAdapter, the appropriate InsertCommand, UpdateCommand, or DeleteCommand is run to push your changes to the database. Let's take a look at how to set the InsertCommand, UpdateCommand, and DeleteCommand properties of a DataAdapter. Setting the InsertCommand Property of a DataAdapter The following example creates a SqlCommand object named myInsertCommand that contains an INSERT statement:
  2. SqlCommand myInsertCommand = mySqlConnection.CreateCommand(); myInsertCommand.CommandText = "INSERT INTO Customers (" + " CustomerID, CompanyName, Address" + ") VALUES (" + " @CustomerID, @CompanyName, @Address" + ")"; myInsertCommand.Parameters.Add("@CustomerID", SqlDbType.NChar, 5, "CustomerID"); myInsertCommand.Parameters.Add("@CompanyName", SqlDbType.NVarChar, 40, "CompanyName"); myInsertCommand.Parameters.Add("@Address", SqlDbType.NVarChar, 60, "Address"); The four parameters to the Add() method are as follows: • The name of the parameter • The .NET type of the parameter • The maximum length of the string that may be assigned to the parameter's value • The name of the corresponding database column that the parameter is bound to Note Commands and parameters are covered in Chapter 8, "Executing Database Commands." As you can see from the previous code, the @CustomerID, @CompanyName, and @Address parameters are bound to the CustomerID, CompanyName, and Address columns in the database. Next, the following example sets the InsertCommand property of mySqlDataAdapter to myInsertCommand: mySqlDataAdapter.InsertCommand = myInsertCommand; Setting the UpdateCommand Property of a DataAdapter The following example creates a SqlCommand object named myUpdateCommand that contains an UPDATE statement and sets the UpdateCommand property of mySqlDataAdapter to myUpdateCommand: myUpdateCommand.CommandText = "UPDATE Customers " + "SET " + " CompanyName = @NewCompanyName, " +
  3. " Address = @NewAddress " + "WHERE CustomerID = @OldCustomerID " + "AND CompanyName = @OldCompanyName " + "AND Address = @OldAddress"; myUpdateCommand.Parameters.Add("@NewCompanyName", SqlDbType.NVarChar, 40, "CompanyName"); myUpdateCommand.Parameters.Add("@NewAddress", SqlDbType.NVarChar, 60, "Address"); myUpdateCommand.Parameters.Add("@OldCustomerID", SqlDbType.NChar, 5, "CustomerID"); myUpdateCommand.Parameters.Add("@OldCompanyName", SqlDbType.NVarChar, 40, "CompanyName"); myUpdateCommand.Parameters.Add("@OldAddress", SqlDbType.NVarChar, 60, "Address"); myUpdateCommand.Parameters["@OldCustomerID"].SourceVersion = DataRowVersion.Original; myUpdateCommand.Parameters["@OldCompanyName"].SourceVersion = DataRowVersion.Original; myUpdateCommand.Parameters["@OldAddress"].SourceVersion = DataRowVersion.Original; mySqlDataAdapter.UpdateCommand = myUpdateCommand; There are two things to notice about this code: • The UPDATE statement's WHERE clause specifies parameters for CompanyID, CompanyName, and Address columns. This uses optimistic concurrency, which you'll learn about shortly. • A property named SourceVersion for the @OldCustomerID, @OldCompanyName and @OldAddress parameters is set to DataRowVersion.Original. This causes the values for these parameters to be set to the original DataRow column values before you change them. These items determine the concurrency of the UPDATE, which you'll now learn about. Concurrency Concurrency determines how multiple users' modifications to the same row are handled. There are two types of concurrency that apply to a DataSet: • Optimistic Concurrency With optimistic concurrency, you can modify a row in a database table only if no one else has modified that same row since you loaded it into your DataSet. This is typically the best type of concurrency to use because you don't want to overwrite someone else's changes.
  4. • "Last One Wins" Concurrency With "last one wins" concurrency, you can always modify a row-and your changes overwrite anyone else's changes. You typically want to avoid using "last one wins" concurrency. To use optimistic concurrency, you have to do the following in your UPDATE or DELETE statement's WHERE clause: 1. Include all the columns used in the original SELECT. 2. Set these column values to original values retrieved from the row in the table before you changed the values. When you do these two things in your UPDATE or DELETE statement's WHERE clause, your statement first checks that the original row still exists before updating or deleting the row. That way, you can be sure your changes don't overwrite anyone else's changes. Of course, if the original row has been deleted by another user, then your UPDATE or DELETE statement will fail. To use "last one wins" concurrency, you just include the primary key and its value in the WHERE clause of your UPDATE or DELETE statement. Since your UPDATE statement doesn't check the original values, it simply overwrites anyone else's changes if the row still exists. Also, a DELETE statement simply deletes the row-even if another user has modified the row. Returning to the previous code example that set the UpdateCommand property of mySqlDataAdapter, you can see that all the columns are included in the WHERE clause of the UPDATE. That satisfies the first requirement of using optimistic concurrency shown earlier. The second requirement is that you set the column in the WHERE clause to the original row values. You do this by setting the SourceVersion property of the @OldCustomerID, @OldCompanyName, and @OldAddress parameters to DataRowVersion.Original. At runtime, this pulls the original values from the DataColumn objects in the DataRow before you changed them and puts them in the UPDATE statement's WHERE clause. Original is just one of the members of the System.Data.DataRowVersion enumeration; the others are shown in Table 11.9. Table 11.9: DataRowVersion ENUMERATION MEMBERS CONSTANT DESCRIPTION Current The current column value. Default The default column value. Original The original column value.
  5. Table 11.9: DataRowVersion ENUMERATION MEMBERS CONSTANT DESCRIPTION Proposed The proposed column value, which is set when you edit a DataRow using the BeginEdit() method. Setting the DeleteCommand Property of a DataAdapter The following example creates a SqlCommand object named myDeleteCommand that contains a DELETE statement and sets the DeleteCommand property of mySqlDataAdapter to myDeleteCommand: SqlCommand myDeleteCommand = mySqlConnection.CreateCommand(); myDeleteCommand.CommandText = "DELETE FROM Customers " + "WHERE CustomerID = @OldCustomerID " + "AND CompanyName = @OldCompanyName " + "AND Address = @OldAddress"; myDeleteCommand.Parameters.Add("@OldCustomerID", SqlDbType.NChar, 5, "CustomerID"); myDeleteCommand.Parameters.Add("@OldCompanyName", SqlDbType.NVarChar, 40, "CompanyName"); myDeleteCommand.Parameters.Add("@OldAddress", SqlDbType.NVarChar, 60, "Address"); myDeleteCommand.Parameters["@OldCustomerID"].SourceVersion = DataRowVersion.Original; myDeleteCommand.Parameters["@OldCompanyName"].SourceVersion = DataRowVersion.Original; myDeleteCommand.Parameters["@OldAddress"].SourceVersion = DataRowVersion.Original; mySqlDataAdapter.DeleteCommand = myDeleteCommand; Notice that the DELETE statement also uses optimistic concurrency. This completes the setup of the DataAdapter object. Adding a DataRow to a DataTable In this section, you'll learn how to add a DataRow to a DataTable. Before you see this, let's populate a DataSet with the rows from the Customers table. The following code creates a DataSet object named myDataSet and populates it by calling mySqlDataAdapter.Fill():
  6. DataSet myDataSet = new DataSet(); mySqlConnection.Open(); int numOfRows = mySqlDataAdapter.Fill(myDataSet, "Customers"); mySqlConnection.Close(); The int returned by the Fill() method is the number of rows retrieved from the database. The myDataSet object now contains a DataTable named Customers, which contains the rows retrieved by the following SELECT statement set earlier in the SelectCommand property of mySqlDataAdapter: SELECT CustomerID, CompanyName, Address FROM Customers ORDER BY CustomerID To add a new row to a DataTable object, you use the following steps: 1. Use the NewRow() method of your DataTable to create a new DataRow. 2. Set the values for the DataColumn objects of your new DataRow. Note: you can set a DataColumn value to null using the SetNull() method of a DataRow. You can also check if a DataColumn contains null using the IsNull() method of a DataRow. 3. Use the Add() method through the Rows property of your DataTable to add your new DataRow to the DataTable. 4. Use the Update() method of your DataAdapter to push the new row to the database. The following method, named AddDataRow(), uses these steps to add a new row to a DataTable: public static void AddDataRow( DataTable myDataTable, SqlDataAdapter mySqlDataAdapter, SqlConnection mySqlConnection ) { Console.WriteLine("\nIn AddDataRow()"); // step 1: use the NewRow() method of the DataTable to // create a new DataRow Console.WriteLine("Calling myDataTable.NewRow()"); DataRow myNewDataRow = myDataTable.NewRow(); Console.WriteLine("myNewDataRow.RowState = " + myNewDataRow.RowState);
  7. // step 2: set the values for the DataColumn objects of // the new DataRow myNewDataRow["CustomerID"] = "J5COM"; myNewDataRow["CompanyName"] = "J5 Company"; myNewDataRow["Address"] = "1 Main Street";
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