Updating the Database Using Data Sets in .NET framework

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Updating the Database Using Data Sets
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So far we have only been looking at retrieving data from a database What happens when you have modified the data and want to save those changes back to the database Well, another powerful capability of the DataSet class is the ability to keep track of changes made to the data contained in it Changes come in three forms You can
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Add new rows to a table Delete rows from a table Change the values in one or several of the columns of existing rows in a table
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The data set handles this through a combination of holding two copies of each row, and through a RowState property on each row, which takes a value from the DataRowState enumeration When you first retrieve a set of rows from the database, they are added to their table with a RowState value of Unchanged When you modify a column value within an existing row, the state is set to Modified When you add a new row or delete an existing one, that row's state will be marked Added or Deleted, respectively And if you create a new row from a table, but have not yet added it to the Rows collection, the row's state will beDetached The row state lets a data adapter detect which rows should be used to perform inserts, updates, or deletes in the database table When you change the values of an existing row, the current values for the columns are kept in one copy of the row, but the original values of that row from when the data was retrieved is also kept in a second copy of the row This allows the original values to be used for optimistic concurrency detection (discussed the next section) The RowState values and their effects are described in Table D2
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Table D2 DataRowState Enumeration Values
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Value Description Original Value Row as filled Row as filled Empty Current Value Row as filled Row with changes Row as created Row as filled Row as created
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Unchanged
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The row has not been modified since the table was filled or since AcceptChanges has been called
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Modified
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One of the column values has been changed since it was filled or since AcceptChanges was called The row is a new row that has been added to the Rows collection since the table was filled or since AcceptChanges was called
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Added
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Deleted
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The row has had the Delete method called on it since the table was filled or since AcceptChanges was called
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Row as filled Empty
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Detached
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The row was created through the table NewRow method, but has not yet been added to the Rows collection
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You update the database with a data set by using a data adapter Keep in mind that when I say "update" the database, I mean execute any update, insert, or delete queries needed Try not to confuse this with the specific SQL UPDATE queries, which only affect
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modified rows and are only one of three forms of "updates" So far, when constructing and using a data adapter to retrieve data, we have been setting its SelectCommand property to an instance of a SqlCommand that wraps a SQL text query or a stored procedure that returns the rows used to initially populate the table To perform updates using a data adapter, you also need to provide command objects for the adapter's InsertCommand, DeleteCommand, and UpdateCommand properties, depending on which of those you expect to perform As you might expect, the InsertCommand will be used for any rows marked as Added, the DeleteCommand for any rows marked as Deleted, and the UpdateCommand for any rows marked as Modified If a data set doesn't contain any rows with a particular row state, the corresponding command doesn't have to be supplied since it won't be used Generally speaking, you will want to provide all three because you cannot be sure that no rows will be in the modified, added, or deleted state when an update is performed Just like the select command used to fill a data set, the commands used for updating the database with a data set can use either SQL text queries or stored procedures, passing the columns of the updated rows as parameters For simple applications that need to work directly against a single table in the database at a time, you can use parameterized SQL text queries For larger scale applications, you may want to use stored procedures Either way, the commands you associate with the data adapter will be executed once for each row in the data set that needs to perform an update This can result in a lot of network round-trips to the database for a large data set that has a lot of changes in it To address this problem, in ADONET 20 the SqlDataAdapter class includes a batch updating feature that will group all of the queries and send them all to the database in one or several chunks To use this feature, you set the UpdateBatchSize property on the SqlDataAdapter to some value other than 1 (the default) If you set the value to 0 (zero), then all of the updates will be batched into a single bunch If you set some positive value greater than 1, the data adapter will batch sets of that many update queries and send them in chunks until all of the updates have completed You do need to keep in mind, though, that when you send a batch of update queries using this approach, they will all execute as a single transaction in SQL Server So if any one row in a batch fails to perform its update, the entire batch will fail To automatically formulate the update queries for working directly against the tables, the SqlCommandBuilder class can examine the select command that is in use by a data adapter and dynamically build insert, update, and delete commands for you It will then populate the command properties of the data adapter with the generated queries The command builder works well for simple data sets that contain tables generated from SELECT statements against a single table with a primary key constraint This all happens by constructing a command builder object and passing in the data adapter that it is to create commands for You won't actually call any of its methods or properties directly The constructor for the command builder reaches into the data adapter and sets its InsertCommand, UpdateCommand, and DeleteCommand properties to suitable queries based on the columns that are populated with the SelectCommand The following code demonstrates the use of a command builder to generate the update queries and use them to push the changes from a data set to the database In this code, the data set and the data adapter are members on the enclosing class because they are used in a different method to populate the data set with a SELECT query private void btnSaveChanges_Click(object sender, SystemEventArgs e) { // Get the connection off the existing select command of the adapter SqlConnection conn = m_AdapterSelectCommandConnection; // Create the insert, update, and delete commands // Simply constructing the command builder generates // and populates those commands on the adapter SqlCommandBuilder cmdBuilder = new SqlCommandBuilder(m_Adapter); // Call the update method on the adapter to save all changes // in the data set to the database m_AdapterUpdate(m_CustomersDataSet,"Customers"); // Refill the dataset to make sure any triggered changes // in the DB are reflected m_AdapterFill(m_CustomersDataSet,"Customers"); }
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Using the SqlCommandBuilder is very handy for simple cases However, it has a number of downsides, and you should avoid it in general for production code in favor of explicitly writing your own SQL statements or using stored procedures The queries generated by
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This document was created by an unregistered ChmMagic, please go to http://wwwbisentercom to register it Thanks the command builder always compare the values of all columns that were retrieved by the SELECT query to detect optimistic concurrency violations, as described in the next section However, this is usually not the best approach This also means the queries carry a lot more data along with them for each update than may be necessary For larger scale systems, working with stored procedures is a better approach for reasons of security, maintainability, and performance See the later section "Updating with Data Sets and Stored Procedures" for more details
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