Tài liệu Determine Which Records Will Be Printed at Runtime pptx

10.5 Determine Which Records Will Be Printed at Runtime
I can use the Select Wizard to help limit the records that I want to be displayed on the
report, but that doesn't do my users much good when they want control over that when
the application is running. How do I determine which records are displayed at runtime?
Technique
To handle this, use the SelectionFormula of the CrystalReportsViewer. When you use the
SelectionFormula to limit your records, you are taking advantage of Crystal Report's
Database optimization. This means that the query you generated by setting the
SelectionFormula and the original record source for the report will be "pushed down" to
the server.
SelectionFormula Syntax
You can see the syntax for the SelectionFormula here:
CrystalReportViewer1.SelectionFormula = "{Table.Field} = value"
Value can be various data types, delimited by the usual delimiters, such as single quotes
for strings.
For the new criteria to take effect, after setting the SelectionFormula, you need to call the
RefreshReport method of the CrystalReportViewer.
Setting Report Options
To get optimal use out of the SelectionFormula, you need to keep some things in mind.
You need to set an option of the report that will ensure it uses the record source indexes if
possible.
To do this, you right-click on the report and choose Report Options from the Report
menu. On the Options page, make sure to check Use Indexes or Server for Speed (see
Figure 10.20).
Figure 10.20. Customize your report by using the Report Options dialog box.

Another tip is to make sure you use indexed fields whenever possible.
Tip
Take some time now to check out some of the other report options.

Steps

Open and run the Visual Basic .NET-Chapter 10 solution. Click on the button labeled
How-To 10.5. You can select Regions from the ComboBox control on the top of the
form; the report then reflects the selection (see Figure 10.21).
1. Create a Windows Form.
2. Drag on a ReportDocument object, and set it to point to the report you created in
How-To 10.1. Then name your report document rdHowTo10_5.
3. Place the controls shown in Figure 10.21 onto the form with the properties set in
Table 10.4.
Table 10.4. Label, Combo, and CrystalReportViewer Controls and Their
Property Settings
Object Property Setting
Label Text Pick a Region
ComboBox Name cboRegions
CrystalReportViewer Name cvwRegionReport
Anchor Top, Bottom, Right, Left
ReportSource rdHowTo10_5
4. Add the code in Listing 10.3 into the Load event of the form. This code performs a
DISTINCT SQL statement to get all of the regions used in the Customers table.
The data adapter then fills dtRegions. Before binding dtRegions to the combo box
cboRegions, a new data row is added to allow the user to specify All Regions.
Listing 10.3 frmHowTo10_5.vb: Populating the Selection Combo Box
Private Sub frmHowTo10_5_Load(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles MyBase.Load

Dim odaRegions As New _
OleDb.OleDbDataAdapter("SELECT DISTINCT Region FROM
Customers",
BuildCnnStr("(local)", "Northwind"))
Dim dtRegions As New DataTable()

odaRegions.Fill(dtRegions)

'-- Add the All Regions feature
Dim drAll As DataRow = dtRegions.NewRow
drAll(0) = "<< ALL Regions >>"
dtRegions.Rows.Add(drAll)


With Me.cboRegions
.DataSource = dtRegions
.ValueMember = "Region"
End With


End Sub
Note
As with a number of other examples, the BuildCnnStr() function
has been created in the modGeneralRoutines module to create a
connection string.
5. Add the report in Listing 10.4 to the SelectedIndexChanged event of cboRegions.
One of the values that is in the Region field of the Customer table is NULL. This
code tests for that first by testing this:
6. Me.cboRegions.SelectedItem(0) Is System.DBNull.Value
If the selected item is NULL, which uses System.DBNull.Value for the
comparison, then the value of IsNull({Customers.Region}) is stored in the
SelectionFormula. If All Reasons was chosen, then the SelectFormula is set to the
empty string so that all items are chosen. Otherwise, the Customers.Region field is
compared to the literal region, supplied by cboRegions. The last task performed is
refreshing the report.
Listing 10.4 frmHowTo10_5.vb: Setting the SelectionFormula Property

Private Sub cboRegions_SelectedIndexChanged(ByVal sender As System.Object,
_
ByVal e As System.EventArgs) _
Handles cboRegions.SelectedIndexChanged

With Me.cvwRegionReport

If Me.cboRegions.SelectedItem(0) Is System.DBNull.Value Then

.SelectionFormula = "IsNull({Customers.Region})"

ElseIf Me.cboRegions.SelectedItem(0) = "<< ALL Regions >>" Then

.SelectionFormula = ""

Else

.SelectionFormula = "{Customers.Region} = '" & _
Me.cboRegions.SelectedItem(0) & "'"
End If

.RefreshReport()

End With

End Sub
Figure 10.21. You can let your user control how much data he sees in the report.

Comments
You can limit the selection displayed on the report in other ways, too. This How-To

presented just one way to get you started. Letting your user choose the records gives him
the perceived control he wants.