vb.net qr code sample s INTRODUCING DATASETS AND DATA ADAPTERS in Visual Basic .NET

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CHAPTER 8 s INTRODUCING DATASETS AND DATA ADAPTERS
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Working with Datasets and Data Adapters
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The dataset constructor is overloaded.
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Dim ds as DataSet = new DataSet() Dim ds as DataSet = new DataSet("MyDataSet")
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If you use the parameterless constructor, the dataset name defaults to NewDataSet. If you need more than one dataset, it s good practice to use the other constructor and name it explicitly. However, you can always change the dataset name by setting its DataSetName property. You can populate a dataset in several ways, including: Using a data adapter Reading from an XML document In this chapter, we ll use data adapters. However, in the Using Datasets and XML section, you ll take a quick peek at the converse of the second method, and you ll write from a dataset to an XML document.
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Try It Out: Populating a Dataset with a Data Adapter
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In this example, you ll create a dataset, populate it with a data adapter, and then display its contents. 1. In VBE, create a new Console Application project named PopDataSet in a new solution named 08. 2. Replace the code in Module1.vb with the code in Listing 8-1.
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Listing 8-1. PopDataSet: Module1.vb
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Imports System Imports System.Data Imports System.Data.SqlClient Module Module1 Sub Main() ' Set up connection string Dim connString As String = _ "server = .\sqlexpress;" _
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CHAPTER 8 s INTRODUCING DATASETS AND DATA ADAPTERS
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& "integrated security = true;" _ & "database = northwind" ' Set up query Dim sql As String = _ "select " _ & " productname, " _ & " unitprice " _ & "from & " " _ products " _
& "where " _ & " unitprice < 20 " ' Create connection Dim conn As SqlConnection = New SqlConnection(connString) Try ' Open connection conn.Open() ' Create data adapter Dim da As SqlDataAdapter = New SqlDataAdapter(sql, conn) ' Create dataset Dim ds As DataSet = New DataSet() ' Fill dataset da.Fill(ds, "products") ' Get table data Dim dt As DataTable = ds.Tables("products") ' Loop through data table For Each row As DataRow In dt.Rows For Each col As DataColumn In dt.Columns Console.WriteLine(row(col)) Next Console.WriteLine("".PadLeft(20, "=")) Next Catch e As Exception ' Display error Console.WriteLine("Error: " & e.ToString)
CHAPTER 8 s INTRODUCING DATASETS AND DATA ADAPTERS
Finally ' Close connection conn.Close() End Try End Sub End Module
3. Run it with Ctrl+F5. You should see the result in Figure 8-3. (Only the last ten rows are displayed.)
Figure 8-3. Populating a dataset
How It Works
After defining a query and opening a connection, you created and initialized a data adapter:
' Create data adapter Dim da As SqlDataAdapter = New SqlDataAdapter(sql, conn)
then created a dataset:
' Create dataset Dim ds As DataSet = New DataSet()
CHAPTER 8 s INTRODUCING DATASETS AND DATA ADAPTERS
At this stage, all you had was an empty dataset. The key line is where you used the Fill method on the data adapter to execute the query, retrieve the data, and populate the dataset:
' Fill dataset da.Fill(ds, "products")
The Fill method uses a data reader internally to access the table schema and data and then use them to populate the dataset. Note that this method isn t just used for filling datasets. It has a number of overloads and can also be used for filling an individual data table without a dataset, if needed. If you don t provide a name for the table to the Fill method, it will automatically be named TableN, where N starts as an empty string (the first table name is simply Table) and increments every time a new table is inserted into the dataset. It s better practice to explicitly name data tables, but here it doesn t really matter. If the same query is run more than once, on the dataset that already contains data, then Fill() updates the data, skipping the process of redefining the table based on the schema. It s worth mentioning here that the following code would have produced the same result. Instead of passing the SQL and connection to the data adapter s constructor, you could have set its SelectCommand property with a command that you create with the appropriate SQL and connection.
' Create data adapter Dim da As SqlDataAdapter = New SqlDataAdapter()
With a populated dataset at your disposal, you can now access the data in individual data tables. (This dataset contains only one data table.)
' Get table data Dim dt As DataTable = ds.Tables("products")
Finally, you use nested For Each loops to access the columns in each row and output their data values to the screen:
' Loop through data table For Each row As DataRow In dt.Rows For Each col As DataColumn In dt.Columns Console.WriteLine(row(col)) Next Console.WriteLine("".PadLeft(20, "=")) Next
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