Tài liệu Limit the Data Displayed in a Bound List Box doc

You will then add a Label and TextBox control that the Select statement contained within the OleDbDataAdapter control will query against to limit the data displayed in the list box.. Res

1.2 Limit the Data Displayed in a Bound List Box

Even populating a list box with a couple of columns from a table full of data can be a big performance hit This How-To shows you how to create a parameterized SQL statement

to limit the items that are displayed in the list box, thus giving you better performance on your forms

You have hundreds of thousands of customers in your database, and you don't want the list box loaded up with the whole customer table How can you limit the data that is displayed in your list box?

Technique

You are going to make a copy of the form that you created in How-To 1.1 You will then add a Label and TextBox control that the Select statement contained within the

OleDbDataAdapter control will query against to limit the data displayed in the list box A command button will be added to allow you to call the Fill method of the

OleDbDataAdapter control whenever you update the text box, and then you can click the command button (see Figure 1.6)

Figure 1.6 You can now limit the amount of data loaded into the list box

Steps

To get started with this How-To, right-click the form you created in How-To 1.1, which should be listed in the Solutions Explorer Choose Copy from the pop-up menu Next, right-click the project in the Solution Explorer, and choose Paste from the pop-up menu You will now have a new Class object in the Solutions Explorer called Copy Of whatever the previous name of the form was Rename the new form that you have created to the

name you desire Then, with that form highlighted, click on the Code button above the

Solutions Explorer Change the first line of code to say this:

Public Class <Name of the new form>

You see, VS does not change the line of code automatically for you It thinks you have a

duplicate Class definition

Now you can see that the icon of the form is correct You can continue with the steps of

the How-To

1 Select the Data Adapter that you created In the Properties pane, you will see the

CommandText property when you click on the SelectCommand property plus

sign Replace the CommandText property with the following command:

2 SELECT CustomerID, CompanyName FROM Customers WHERE

(CompanyName LIKE ? + '%')

You will learn more about the Select statement in Chapter 3 However, the

WHERE clause used here compares CompanyName to a parameter that will be

supplied, as indicated by the ? This will be performed using code in the final step

of this How-To The % is a wildcard that tells the server to make it a fuzzy search

3 Resize the ListBox control, and leave room at the top of the form for the Label,

TextBox, and Command button Create these three controls, setting the properties

described in Table 1.3

Table 1.3 Label, TextBox, and Command Button Control Property Settings

Command Button Name btnLoadList

4 Double-click the new command button you just created called btnLoadList Enter

the code in Listing 1.2 in the Click event of the btnLoadList button This code

loads the data entered from txtCustLimit into the parameter of the

OleDBDataAdapter1, which was created by using the ? in the Select statement of

the data adapter Then Dataset1 is cleared of its data with the Clear method

Finally, DataSet1 is refilled with data based off the value in txtCustLimit, using the data adapter

Listing 1.2 frmHowTo1_2.vb: Submitting a Parameter to a DataAdapter and Filling the Dataset

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

ByVal e As System.EventArgs) Handles btnLoadList.Click

Me.OleDbDataAdapter1.SelectCommand.Parameters(0).Value = _

Me.txtCustLimit.Text

Me.DataSet1.Clear()

Me.OleDbDataAdapter1.Fill(Me.DataSet1)

End Sub

Note

There is one big difference here between an OleDbDataAdapter and a SqlDataAdapter Whereas the OleDbDataAdapter takes a ?

to specify a parameter within the Select statement, the SqlDataAdapter requires a named parameter such as

@parCustLimit Therefore, instead of the select statement in step

1 being this:

SELECT CustomerID, CompanyName FROM Customers WHERE (CompanyName LIKE ? + '%')

It would be this:

SELECT CustomerID, CompanyName FROM Customers WHERE (CompanyName LIKE @parCustLimit + '%') The naming of the actual parameter is up to you

5 Highlight and delete the Load event for the form because you don't want to just fill the event when the form loads You want the user to click btnLoadList

How It Works

When the form you created for this How-To loads, or when you're using the form called frmHowTo1_2, you will see a blank ListBox control with a text box on top with the letter

A in it If you click the command button called btnLoadList, the list box becomes filled with values based on the letter (or letters) in txtCustLimit and on the code described in step 3

Comments

Try entering a few other letters, and then try entering no letters What happens? You limit the list more by the number of letters you enter, and you get all entries when you don't enter any letters

The method displayed here, although simple, is powerful, and it can be used in a variety

of ways

You can continue building on this form for the next few How-Tos If you want to copy your form and start a new one as described at the beginning of the steps for this one, you have the instructions there Otherwise, by the time you reach How-To 1.8, you will have

a data entry form that you can use Each step, however, is available in the sample solution for this chapter