Tài liệu Filter and Sort Records Using the DataView Object pptx

3.4 Filter and Sort Records Using the DataView Object After your data is loaded into the data table, you will probably want to be able to view your data using different filters and sort

3.4 Filter and Sort Records Using the DataView Object

After your data is loaded into the data table, you will probably want to be able to view your data using different filters and sort orders To do this, you can use the DataView object This How-To goes into detail and shows you how to take advantage of the

DataView control to manipulate your data

Although you can put data into the DataGrid control and let the users sort data using the columns, you want to display a ComboBox control and let users pick a field from the drop-drown list How can you filter and sort records using the DataView object to present your data in different ways?

Technique

This How-To displays a set of command buttons that display a letter and an extra

command button that displays all records A data adapter, data table, and data view are declared at the form level The data adapter is created and the DataTable is filled when the form is loaded with all customers Using a DataColumn object, a combo box is filled

by getting the names of each column that is in the data table You can see this form in action in Figure 3.5

Figure 3.5 Selecting a letter here limits the data displayed in the DataGrid control

Using the command buttons, a routine is called that creates a DataView object, sets the RowFilter property, and then assigns the data view to the DataSource property of a DataGrid control

Tip

Although the RowFilter allows you to filter data based on a criteria such

as CompanyName Like 'A%', you can set another property to display data based on the state of the row in which data occurs The property is called RowStateFilter

You can set the RowStateFilter to one of the following DataViewRowState values in Table 3.5

Table 3.5 Label, TextBox, and ComboBox Control Property Settings

Setting Description

Added New rows

CurrentRows Current rows including unchanged, new, and modified rows

ModifiedCurrent A current version, which is a modified version of original data (see

ModifiedOriginal) ModifiedOriginal The original version (although it has since been modified and is

available as ModifiedCurrent) None None

OriginalRows Original rows including unchanged and deleted rows

The Sort property of the DataView object is used when a column name is chosen from the ComboBox The current setting of the Sort property is compared to the column name that is chosen If the Name matches, then the expression DESC is added to the value that

is assigned to the Sort property

Steps

Open and run the VB.NET-Chapter 3 solution From the main form, click on the

command button with the caption How-To 3.4 When the form loads, click on different letters that are displayed You will see the data grid display different customers based on their first letter

If you choose a column name from the Column to Sort On ComboBox control, the data grid will then be sorted based on the column chosen

1 Create a new Windows Form

2 Add a GroupBox control with the text property set to Click on a Letter

3 Now you will be creating buttons that you will place within the GroupBox control you just created The buttons will have their property set as listed in Table 3.6

Table 3.6 Buttons Property Settings Object Property Setting

Button Name btnB

4 Add the DataGrid, the Label, and the ComboBox controls shown in Listing 3.6

Table 3.7 DataGrid, Label, and ComboBox Controls Property Settings

DataGrid Name dgCustomers

5 In the class module for the form, add the following three Private declarations just below the line of code that reads Windows Form Designer generated code These three objects will be used throughout the form

6 Private modaCust As OleDb.OleDbDataAdapter

7 Private mdtCust As DataTable = New DataTable()

8 Private mdvCust As DataView = New DataView()

9 Add the following code to the Load event of the form as shown in Listing 3.6 This code starts out by setting up the modaCust data adapter to grab all the

customers to fill the data table called mdtCust Note that at this point, the data grid has not been filled

The next task is to load cboSortColumns with the column headings by iterating through each of the data columns in mdtCust and adding them to the Items

collection in cboSortColumns Last, the SetDataViewFilter routine is called This routine is discussed in step 8

Listing 3.6 frmHowTo3_4.vb: Loading the Data Table to Be Used in the Form, and Adding Column Names to a ComboBox Control

Private Sub frmHowTo3_4_Load(ByVal sender As Object, _

ByVal e As System.EventArgs) Handles MyBase.Load

Dim strSQL As String

Dim dcCurr As DataColumn

' Set up the exception catch

Try

' Create the data adapter and fill the data table

modaCust = New _

OleDb.OleDbDataAdapter("Select * From Customers", _

(BuildCnnStr("(local)", "Northwind")))

modaCust.Fill(mdtCust)

' Load the column names into the sort ComboBox control

For Each dcCurr In mdtCust.Columns

Me.cboSortColumns.Items.Add(dcCurr.ColumnName)

Next

SetDataViewFilter("B")

Catch oexpData As OleDb.OleDbException

MsgBox(oexpData.Message)

End Try

End Sub

10 For each of the command buttons that has a single letter, add the first subroutine displayed here in Listing 3.7 to each of its Click events For the btnAll Button control, add the second subroutine to the Click event Each Button control will pass the letter that it represents to the subroutine called SetDataViewFilter,

discussed in the next step The btnAll code simply passes the empty string

Listing 3.7 frmHowTo3_4.vb: Click Events for Each of the Button Controls

Private Sub btnA_Click(ByVal sender As System.Object, _

ByVal e As System.EventArgs) Handles btnA.Click

SetDataViewFilter("A")

End Sub

Private Sub btnAll_Click(ByVal sender As System.Object, _

ByVal e As System.EventArgs) Handles btnAll.Click

SetDataViewFilter("")

End Sub

11 Add the subroutine that is found in Listing 3.8 to the class module of the form This routine takes the letter value that is passed in strFilterLetter as a parameter The first task to perform is assigning the DefaultView of the mdtCust DataTable object to the mdvCust data view Next, the RowFilter property of mdvCust is set

to compare the CompanyName column with the Like expression using the

strFilterLetter and the % (wildcard) Note that if "" is passed to strFilterLetter, all the records will be listed Finally, mdvCust is set as the DataSource for

dgCustomers, which is the DataGrid control

Listing 3.8 frmHowTo3_4.vb: Setting the RowFilter Property for a DataView Object

Sub SetDataViewFilter(ByVal strFilterLetter As String)

mdvCust = mdtCust.DefaultView

mdvCust.RowFilter = "CompanyName Like '" & strFilterLetter & "%'" dgCustomers.DataSource = mdvCust

End Sub

12 Add the piece of code that is shown in Listing 3.9 to the SelectdIndexChanged event of the cboSortColumns ComboBox control This routine compares the current setting of mdvCust's Sort property to the current column name chosen in cboSortColumns If the two are the same, then the column name is assigned to the Sort property with the DESC keyword added on If not, then the name of the column is assigned to the Sort property

Listing 3.9 frmHowTo3_4.vb: Specifying a Column on Which to Sort

Private Sub cboSortColumns_SelectedIndexChanged(ByVal sender As

System.Object,

ByVal e As System.EventArgs) _

Handles cboSortColumns.SelectedIndexChanged

' Check to see if the column is currently the sorted field

' If it is, sort on the column in descending order

' Otherwise, set the sort to the name of column

If mdvCust.Sort = Me.cboSortColumns.Text Then

mdvCust.Sort = Me.cboSortColumns.Text & " DESC"

Else

mdvCust.Sort = Me.cboSortColumns.Text

End If

End Sub

How It Works

When the user clicks on a letter, the data view is created, and the data grid reflects the new data When a field is selected from the ComboBox control, the Sort property of the data view is set and the data grid automatically reflects the new sort order, also showing

an arrow in the column heading If the user chooses the field again, the column will sort

in descending order

Comments

Using the DataView object, you can keep track of multiple views of your data and

display them for the users' use You can also access all of the default views of the data tables in your data set using the DefaultViewManager

Note

Some people might think that the sorting combo box that was added to this example is unnecessary It was added for two reasons First, it shows how to use the Sort property of a DataView object Second, it's

convenient for the user The user might not want to have to scroll over to

a column that is not displayed in the data grid By using the combo box,

he can sort on fields that are not currently displayed