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In the previous example, you saw how to dynamically filter and sort data in a data table using the Select method. However, ADO.NET has another approach for doing much the same thing and more: data views. A data view (an instance of class System.Data.DataView) enables you to create dynamic views of the data stored in an underlying data table, reflecting all the changes made to its content and its ordering. This differs from the Select method, which returns an array of data rows whose contents reflect the changes to data values but not the data ordering.

The Ch04NullToZero.sql file illustrates how to define a default value for a bit column that represents missing values as zeroes rather than the default of null values. The CREATE TABLE statement from the file shows the syntax for the DEFAULT keyword. There is no need for any parentheses after the DEFAULT keyword to designate the constant value (0).

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CREATE TABLE T ( int1 int, bit1 bit NOT NULL DEFAULT 0, varchar1 varchar(3), dec1 dec(5,2), cmp1 AS (int1 + bit1) ) The script in NullToZero.sql includes the same four INSERT statements and the first SELECT statement from Ch04InsertWithMissingValues.sql. When you run these statements after the preceding CREATE TABLE statement, the following listing appears. All four rows appear in the output from the SELECT statement. The DEFAULT 0 constraint for the bit1 column in the CREATE TABLE statement assigns a value of 0 to rows with a missing value namely, the second and third rows. The NOT NULL constraint is not strictly necessary because the DEFAULT constraint overrides it, but the inclusion of the NOT NULL phrase does not cause a compilation error.

s Note A data view is a dynamic representation of the contents of a data table. Like a SQL view, it doesn t

Exercise 2-6

If you have prior experience with earlier versions of SQL Server, you may be familiar with severity levels for errors. Error levels are reported after one or more batches of T-SQL statements run; the error level numbers tell you that something went wrong and how severe it is after the T-SQL code runs. The SSE query tool based on SQL Server Management Studio introduces the notion of a compilation error for T-SQL code. A compilation error is detected by the SSE query tool before batch statements run. The query tool marks lines with syntax or semantic errors. As noted, the NOT NULL and DEFAULT 0 column properties for constraining bit1 column values do not conflict with one another. However, replacing NOT NULL with NULL in the Ch04InsertWithMissingValues.sql file generates a compilation error because a column cannot have a default value of zero and allow null values. If you change NOT NULL to NULL, the SSE query tool marks the changed line.

Try It Out: Refining Data with a Data View We won t cover all aspects of data views here, as they re beyond the scope of this book. However, to show how they can be used, we ll present a short example that uses a data view to dynamically sort and filter an underlying data table. 1. Add a new Console Application project named DataViews to your 08 solution. 2. Replace the code in Module1.vb with the code in Listing 8-3.

bit1 ----0 0 0 1

Imports System Imports System.Data Imports System.Data.SqlClient Module Module1 Sub Main() ' Set up connection string Dim connString As String = _ "server = .\sqlexpress;" _ & "integrated security = true;" _ & "database = northwind" ' Set up query Dim sql As String = _ "select " _ & " contactname, " _ & " country " _ & "from " _ & " customers " ' Create connection Dim conn As SqlConnection = New SqlConnection(connString) Try ' Create data adapter Dim da As SqlDataAdapter = New SqlDataAdapter(sql, conn) ' Create and fill dataset Dim ds As DataSet = New DataSet() da.Fill(ds, "customers") ' Get the data table reference Dim dt As DataTable = ds.Tables("customers") ' Create data view Dim dv As DataView = New DataView( _ dt, _ "country = 'Germany'", _ "country", _ DataViewRowState.CurrentRows)

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