Tài liệu Retrieve Unique Records Using Only a Select Query docx

6.1 Retrieve Unique Records Using Only a Select Query I need to figure out which customers have invoices.. Technique For this How-To, you will be using the DISTINCT clause on a SQL SELE

6.1 Retrieve Unique Records Using Only a Select Query

I need to figure out which customers have invoices The problem is that when I join the Customers with the Orders tables, I get the customers listed for each order I only want each customer listed once How do I return only those customers who have orders, but only once?

Technique

For this How-To, you will be using the DISTINCT clause on a SQL SELECT statement

to limit the data to unique values When you include the DISTINCT clause, SQL Server uses the columns that are returned to determine how to limit the data

For the opposite affect, you can include the ALL clause, although it is not necessary because this is the default You will create two SELECT statements for this task The first one is for all records:

SELECT Customers.CompanyName FROM Customers INNER JOIN Orders ON

Customers CustomerID =

Orders.CustomerID

To limit the records, use the DISTINCT clause:

SELECT DISTINCT Customers.CompanyName

FROM Customers

INNER JOIN Orders ON Customers.CustomerID = Orders.CustomerID

Steps

Open and run the Visual Basic NET-Chapter 6 solution From the main form, click on the button with the caption How-To 6.1 When the form loads, you will see two option buttons, Show All and Distinct, with Show All selected The SELECT statement showing

an inner join between customers and order is displayed in a Label control You will also see a DataGrid control filled with multiple entries of customers displayed (see Figure 6.1)

Figure 6.1 A common problem with inner joins is retrieving multiple records when

you want to see just one per occurrence

If you click on the option button labeled Use Distinct, then the DataGrid control will be refreshed, but only one customer per set of orders will be displayed

1 Create a Windows Form Then place the controls listed in Table 6.1 with the following properties set, as displayed in Figures 6.1 and 6.2

Figure 6.2 Using the DISTINCT clause gives you control over displaying

unique records

Table 6.1 Control Property Settings for How-To 6.1

DataGrid Name dgResults

2 As with some of the other chapters' projects, you need to build a support routine to create the Connection string Called BuildCnnStr, the function can been seen in Listing 6.1 This function takes a server and database name passed to it and creates

a connection string

Listing 6.1 modGeneralRoutines.vb: Creating a Connection String

Function BuildCnnStr(ByVal strServer As String, _

ByVal strDatabase As String) As String

Dim strTemp As String

strTemp = "Provider=SQLOleDB; Data Source=" & strServer & ";"

strTemp &= "Initial Catalog=" & strDatabase & ";"

strTemp &= "Integrated Security=SSPI"

Return strTemp

End Function

Although you could create a routine that would pass back a Connection object, a more versatile method would be to pass back a string The reason for this is that for some objects, you are asked only for a Connection object, but other objects want just a string

3 Add the code in Listing 6.2 to the Load event of the form (Double-click on the form to bring up the code.)

Listing 6.2 frmHowTo6_1.vb: Loading the Form

Private Sub frmHowTo6_1_Load(ByVal sender As System.Object,

ByVal e As System.EventArgs) Handles MyBase.Load

GenerateData(Me.rbDistinct.Checked)

End Sub

4 Add the code in Listing 6.3 to the class module of the page, creating the

GenerateData routine This routine creates the necessary SQL SELECT statement based on whether blnUseDistinct is true or false If you look back at Listing 6.2, you will see that this is the value of option button rbDistinct After the SQL string

is created, it is assigned to lblSQLString to display the string, and then it is used in

a data adapter to fill a dataset Last, the SQL string is assigned as the data source for the data grid dgResults

Listing 6.3 frmHowTo6_1.vb: Building the SQL String for Retrieving the Data

Sub GenerateData(ByVal blnUseDistinct As Boolean)

' Build the SQL String

Dim strSQL As String

strSQL = "SELECT "

If blnUseDistinct Then

strSQL += "DISTINCT "

End If

strSQL += "Customers.CompanyName FROM Customers "

strSQL += "INNER JOIN Orders ON Customers.CustomerID =

Orders.CustomerID"

' Store the SQL String

Me.lblSQLString.Text = strSQL

' Use the SQL String to build the data adapter and fill the data table

Dim odaResults As New OleDb.OleDbDataAdapter(Me.lblSQLString.Text, _

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

Dim dtResults As New DataTable()

odaResults.Fill(dtResults)

' Assign the data table to the data grid's DataSource property

Me.dgResults.DataSource = dtResults

End Sub

5 Add the code in Listing 5.4 to the CheckChanged event of the rbDistinct Radio Button control

Listing 6.4 frmHowTo6_1.vb: Regenerating the Data Based on the Radio Button That Is Checked

Private Sub rbDistinct_CheckedChanged(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles rbDistinct.CheckedChanged

GenerateData(Me.rbDistinct.Checked)

End Sub

Tip

You might have noticed that besides the loading of the form, I only call the GenerateData routine when the rbDistinct option button is changed, and not when the rbShowAll option button is changed Because only two buttons are available, you only have to program one of the control's events If you put it on both, you will have the routine called twice, which is not a good thing in this case

Comments

It is hard to believe that just one word can affect the data that a SQL statement returns For the most part, you will want to see all of the records that a SELECT statement returns, but it is nice to have the DISTINCT clause when you need to limit the data