Populating a DataSet from a DataAdapter
The ADO.NET DataSet is a memory-resident representation of data that provides a consistent relational programming model independent of the data source. The DataSet represents a complete set of data that includes tables, constraints, and relationships among the tables. Because the DataSet is independent of the data source, a DataSet can include data local to the application, and data from multiple data sources. Interaction with existing data sources is controlled through the DataAdapter.
The SelectCommand property of the DataAdapter is a Command object that retrieves data from the data source. The InsertCommand, UpdateCommand, and DeleteCommand properties of the DataAdapter are Command objects that manage updates to the data in the data source according to modifications made to the data in the DataSet. These properties are covered in more detail in Updating Data Sources with DataAdapters.
The Fill method of the DataAdapter is used to populate a DataSet with the results of the SelectCommand of the DataAdapter. Fill takes as its arguments a DataSet to be populated, and a DataTable object, or the name of the DataTable to be filled with the rows returned from the SelectCommand.
Note
Using the DataAdapter to retrieve all of a table takes time, especially if there are many rows in the table. This is because accessing the database, locating and processing the data, and then transferring the data to the client is time-consuming. Pulling all of the table to the client also locks all of the rows on the server. To improve performance, you can use the WHERE clause to greatly reduce the number of rows returned to the client. You can also reduce the amount of data returned to the client by only explicitly listing required columns in the SELECT statement. Another good workaround is to retrieve the rows in batches (such as several hundred rows at a time) and only retrieve the next batch when the client is finished with the current batch.
The Fill method uses the DataReader object implicitly to return the column names and types that are used to create the tables in the DataSet, and the data to populate the rows of the tables in the DataSet. Tables and columns are only created if they do not already exist; otherwise Fill uses the existing DataSet schema. Column types are created as .NET Framework types according to the tables in Data Type Mappings in ADO.NET. Primary keys are not created unless they exist in the data source and DataAdapter.MissingSchemaAction is set to MissingSchemaAction.AddWithKey. If Fill finds that a primary key exists for a table, it will overwrite data in the DataSet with data from the data source for rows where the primary key column values match those of the row returned from the data source. If no primary key is found, the data is appended to the tables in the DataSet. Fill uses any mappings that may exist when you populate the DataSet (see DataAdapter DataTable and DataColumn Mappings).
Note
If the SelectCommand returns the results of an OUTER JOIN, the DataAdapter does not set a PrimaryKey value for the resulting DataTable. You must define the PrimaryKey yourself to make sure that duplicate rows are resolved correctly. For more information, see Defining Primary Keys.
The following code example creates an instance of a SqlDataAdapter that uses a SqlConnection to the Microsoft SQL Server Northwind database and populates a DataTable in a DataSet with the list of customers. The SQL statement and SqlConnection arguments passed to the SqlDataAdapter constructor are used to create the SelectCommand property of the SqlDataAdapter.
Example
' Assumes that connection is a valid SqlConnection object.
Dim queryString As String = _
"SELECT CustomerID, CompanyName FROM dbo.Customers"
Dim adapter As SqlDataAdapter = New SqlDataAdapter( _
queryString, connection)
Dim customers As DataSet = New DataSet
adapter.Fill(customers, "Customers")
// Assumes that connection is a valid SqlConnection object.
string queryString =
"SELECT CustomerID, CompanyName FROM dbo.Customers";
SqlDataAdapter adapter = new SqlDataAdapter(queryString, connection);
DataSet customers = new DataSet();
adapter.Fill(customers, "Customers");
Note
The code shown in this example does not explicitly open and close the Connection. The Fill method implicitly opens the Connection that the DataAdapter is using if it finds that the connection is not already open. If Fill opened the connection, it also closes the connection when Fill is finished. This can simplify your code when you deal with a single operation such as a Fill or an Update. However, if you are performing multiple operations that require an open connection, you can improve the performance of your application by explicitly calling the Open method of the Connection, performing the operations against the data source, and then calling the Close method of the Connection. You should try to keep connections to the data source open as briefly as possible to free resources for use by other client applications.
Multiple Result Sets
If the DataAdapter encounters multiple result sets, it creates multiple tables in the DataSet. The tables are given an incremental default name of TableN, starting with "Table" for Table0. If a table name is passed as an argument to the Fill method, the tables are given an incremental default name of TableNameN, starting with "TableName" for TableName0.
Populating a DataSet from Multiple DataAdapters
Any number of DataAdapter objects can be used with a DataSet. Each DataAdapter can be used to fill one or more DataTable objects and resolve updates back to the relevant data source. DataRelation and Constraint objects can be added to the DataSet locally, which enables you to relate data from dissimilar data sources. For example, a DataSet can contain data from a Microsoft SQL Server database, an IBM DB2 database exposed through OLE DB, and a data source that streams XML. One or more DataAdapter objects can handle communication to each data source.
Example
The following code example populates a list of customers from the Northwind database on Microsoft SQL Server, and a list of orders from the Northwind database stored in Microsoft Access 2000. The filled tables are related with a DataRelation, and the list of customers is then displayed with the orders for that customer. For more information about DataRelation objects, see Adding DataRelations and Navigating DataRelations.
' Assumes that customerConnection is a valid SqlConnection object.
' Assumes that orderConnection is a valid OleDbConnection object.
Dim custAdapter As SqlDataAdapter = New SqlDataAdapter( _
"SELECT * FROM dbo.Customers", customerConnection)
Dim ordAdapter As OleDbDataAdapter = New OleDbDataAdapter( _
"SELECT * FROM Orders", orderConnection)
Dim customerOrders As DataSet = New DataSet()
custAdapter.Fill(customerOrders, "Customers")
ordAdapter.Fill(customerOrders, "Orders")
Dim relation As DataRelation = _
customerOrders.Relations.Add("CustOrders", _
customerOrders.Tables("Customers").Columns("CustomerID"), _
customerOrders.Tables("Orders").Columns("CustomerID"))
Dim pRow, cRow As DataRow
For Each pRow In customerOrders.Tables("Customers").Rows
Console.WriteLine(pRow("CustomerID").ToString())
For Each cRow In pRow.GetChildRows(relation)
Console.WriteLine(vbTab & cRow("OrderID").ToString())
Next
Next
// Assumes that customerConnection is a valid SqlConnection object.
// Assumes that orderConnection is a valid OleDbConnection object.
SqlDataAdapter custAdapter = new SqlDataAdapter(
"SELECT * FROM dbo.Customers", customerConnection);
OleDbDataAdapter ordAdapter = new OleDbDataAdapter(
"SELECT * FROM Orders", orderConnection);
DataSet customerOrders = new DataSet();
custAdapter.Fill(customerOrders, "Customers");
ordAdapter.Fill(customerOrders, "Orders");
DataRelation relation = customerOrders.Relations.Add("CustOrders",
customerOrders.Tables["Customers"].Columns["CustomerID"],
customerOrders.Tables["Orders"].Columns["CustomerID"]);
foreach (DataRow pRow in customerOrders.Tables["Customers"].Rows)
{
Console.WriteLine(pRow["CustomerID"]);
foreach (DataRow cRow in pRow.GetChildRows(relation))
Console.WriteLine("\t" + cRow["OrderID"]);
}
SQL Server Decimal Type
By default, the DataSet stores data by using .NET Framework data types. For most applications, these provide a convenient representation of data source information. However, this representation may cause a problem when the data type in the data source is a SQL Server decimal or numeric data type. The .NET Framework decimal data type allows a maximum of 28 significant digits, whereas the SQL Server decimal data type allows 38 significant digits. If the SqlDataAdapter determines during a Fill operation that the precision of a SQL Server decimal field is larger than 28 characters, the current row is not added to the DataTable. Instead the FillError event occurs, which enables you to determine whether a loss of precision will occur, and respond appropriately. For more information about the FillError event, see Handling DataAdapter Events. To get the SQL Server decimal value, you can also use a SqlDataReader object and call the GetSqlDecimal method.
ADO.NET 2.0 introduced enhanced support for System.Data.SqlTypes in the DataSet. For more information, see SqlTypes and the DataSet.
OLE DB Chapters
Hierarchical rowsets, or chapters (OLE DB type DBTYPE_HCHAPTER, ADO type adChapter) can be used to fill the contents of a DataSet. When the OleDbDataAdapter encounters a chaptered column during a Fill operation, a DataTable is created for the chaptered column, and that table is filled with the columns and rows from the chapter. The table created for the chaptered column is named by using both the parent table name and the chaptered column name in the form "ParentTableNameChapteredColumnName". If a table already exists in the DataSet that matches the name of the chaptered column, the current table is filled with the chapter data. If there is no column in an existing table that matches a column found in the chapter, a new column is added.
Before the tables in the DataSet are filled with the data in the chaptered columns, a relation is created between the parent and child tables of the hierarchical rowset by adding an integer column to both the parent and child table, setting the parent column to auto-increment, and creating a DataRelation using the added columns from both tables. The added relation is named by using the parent table and chapter column names in the form "ParentTableNameChapterColumnName".
Note that the related column only exists in the DataSet. Subsequent fills from the data source can cause new rows to be added to the tables instead of changes being merged into existing rows.
Note also that, if you use the DataAdapter.Fill overload that takes a DataTable, only that table will be filled. An auto-incrementing integer column will still be added to the table, but no child table will be created or filled, and no relation will be created.
The following example uses the MSDataShape Provider to generate a chapter column of orders for each customer in a list of customers. A DataSet is then filled with the data.
Using connection As OleDbConnection = New OleDbConnection( _
"Provider=MSDataShape;Data Provider=SQLOLEDB;" & _
"Data Source=(local);Integrated " & _
"Security=SSPI;Initial Catalog=northwind")
Dim adapter As OleDbDataAdapter = New OleDbDataAdapter( _
"SHAPE {SELECT CustomerID, CompanyName FROM Customers} " & _
"APPEND ({SELECT CustomerID, OrderID FROM Orders} AS Orders " & _
"RELATE CustomerID TO CustomerID)", connection)
Dim customers As DataSet = New DataSet()
adapter.Fill(customers, "Customers")
End Using
using (OleDbConnection connection = new OleDbConnection("Provider=MSDataShape;Data Provider=SQLOLEDB;" +
"Data Source=(local);Integrated Security=SSPI;Initial Catalog=northwind"))
{
OleDbDataAdapter adapter = new OleDbDataAdapter("SHAPE {SELECT CustomerID, CompanyName FROM Customers} " +
"APPEND ({SELECT CustomerID, OrderID FROM Orders} AS Orders " +
"RELATE CustomerID TO CustomerID)", connection);
DataSet customers = new DataSet();
adapter.Fill(customers, "Customers");
}
When the Fill operation is complete, the DataSet contains two tables: Customers and CustomersOrders, where CustomersOrders represents the chaptered column. An additional column named Orders is added to the Customers table, and an additional column named CustomersOrders is added to the CustomersOrders table. The Orders column in the Customers table is set to auto-increment. A DataRelation, CustomersOrders, is created by using the columns that were added to the tables with Customers as the parent table. The following tables show some sample results.
TableName: Customers
| CustomerID | CompanyName | Orders |
|---|---|---|
| ALFKI | Alfreds Futterkiste | 0 |
| ANATR | Ana Trujillo Emparedados y helados | 1 |
TableName: CustomersOrders
| CustomerID | OrderID | CustomersOrders |
|---|---|---|
| ALFKI | 10643 | 0 |
| ALFKI | 10692 | 0 |
| ANATR | 10308 | 1 |
| ANATR | 10625 | 1 |