A group blog from members of the VB team
Several people asked me whether TableAdapter can be used to retrieve multiple result sets from stored procedures. The quick answer is... No, you can't do that with TableAdapter.Fill() method that gets generated as a part of Typed Dataset. But there's a very simple work-around.
DataAdapter.Fill() & Multiple Result Sets
TableAdapter.Fill() method calls DataAdapter.Fill() to retrieve data from database. And DataSet.Fill() method allows you to retrieve multiple result sets from a stored procedure. In order to retrieve multiple result sets, an overload of DataAdapter.Fill() that takes Dataset as a parameter needs to be used. If you do that, you get back a Dataset that contains multiple data tables filled with result sets returned by your stored procedure.
Here's a simple example that shows you how this works.
Let's say that you have a stored procedure called dbo.spSelectCustomersOrders in Northwind database.
CREATE PROCEDURE spSelectCustomersOrders AS BEGIN SET NOCOUNT ON; SELECT * FROM Customers SELECT * FROM Orders END GO
Below code will call this stored procedure and fill Dataset with both result sets.
Dim myConn As New System.Data.SqlClient.SqlConnection Dim myAdapter As New System.Data.SqlClient.SqlDataAdapter Dim mySelectCommand As New System.Data.SqlClient.SqlCommand Dim myDataset As New System.Data.DataSet myConn.ConnectionString = "Data Source=.\SQLExpress;Initial Catalog=Northwind;Integrated Security=True" mySelectCommand.Connection = myConn mySelectCommand.CommandText = "dbo.spSelectCustomersOrders" myAdapter.SelectCommand = mySelectCommand myAdapter.Fill(myDataset) For Each table As System.Data.DataTable In myDataset.Tables Console.WriteLine("Table Name:" & table.TableName) Next
Output of this code looks like this.
Table Name: TableTable Name: Table1
As you can see, DataAdapter.Fill() method executed the stored procedure and put each result set into two separate DataTables.
But why doesn't TableAdapter.Fill() correctly handle multiple result sets? It's because TableAdapter.Fill() calls DataAdapter.Fill() overload that takes DataTable, not Dataset. This can be easily addressed by creating a new Fill method in TableAdapter that calls DataAdapter.Fill() overload that takes Dataset.
Assume we have NorthwindDataset.xsd that contains both Customers and Orders. And let's use the above stored procedure for our new Fill method. Add following code to your partial class file. You can create a partial class file by either double-clicking on the Dataset Designer surface, right-click on the Dataset Designer surface and choose "View Code" or add an empty class file manually.
Namespace NorthwindDataSetTableAdapters Partial Public Class CustomersTableAdapter Public Function FillCustomersOrders(ByVal dataSet As NorthwindDataSet) As Integer Dim multiSelectCommand As New System.Data.SqlClient.SqlCommand Dim returnValue As Integer multiSelectCommand.Connection = Me.Connection multiSelectCommand.CommandText = "dbo.spSelectCustomersOrders" Me.Adapter.SelectCommand = multiSelectCommand '' Map auto-created Table1 that holds the second result-set (Orders rows) to '' Orders DataTable in our Dataset. Me.Adapter.TableMappings.Add("Table1", "Orders") returnValue = Me.Adapter.Fill(dataSet) Return returnValue End Function End Class End Namespace
Couple of key points to note.
First, this new FillCustomersOrders takes Dataset as a parameter. This way, when we call DataAdapter.Fill() we can retrieve both result sets and fill Dataset appropriately.
Second, notice how we use TableMapping to map automatically created DataTable to Orders DataTable in our Dataset. When result sets are retrieved by DataAdapter.Fill() method, each result set is stored in separate DataTables in Dataset. By default, these DataTables are named as Table, Table1, Table2, etc. In order to map these DataTables to typed DataTables defined in our Dataset, we need to use TableMapping. If you open up code behind file for NorthwindDataset.xsd, for example, and browse to InitAdapter() method of one of TableAdapter class, you will see something similar to below.
tableMapping.SourceTable = "Table" tableMapping.DataSetTable = "Customers" '' Colum mapping code skipped ... Me._adapter.TableMappings.Add(tableMapping)
This block of code ensures that what gets returned by DataAdapter.Fill method maps to the correct typed DataTable in our Dataset. In our case with FillCustomersOrders, we know that the second result set contains Orders rows. So we need to create a mapping between Table1 and Orders to make sure Orders DataTable gets filled correctly.
After you add above code to the partial class file, you can call FillCustomersOrders to fill both Customers and Orders with just one round-trip to database.
I can see how this kind of capability could be helpful in some cases. But you have to remember that this comes with its own issues. Although you might think that avoiding multiple trips to database would help in terms of performance, you could end up hurting performance by retrieving large set of data when you only need a sub-set. A lot of times, it is much better to retrieve smaller set of data, only what you need at the moment, and make another call to database when you need other sets of data. ADO.NET does a pretty good job of optimizing connection pooling so making multiple connections to database doesn't cause performance bottleneck in many cases. Rule of thumb. Only retrieve what you need.
But I can see how retrieving multiple result sets can be helpful in some cases. So, make use of the technique I described here and just make sure you consider performance issues always.
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