VB แนะนำด้วยครับงมมาได้แค่นี้ล่ะไปต่อไม่เป็นระบบ Login กับป้องกันการ Insert ข้อมูลซ้ำครับ

 If TextBox1.Text = "" And TextBox2.Text = "" Then
            MsgBox("กรุณาระบุ Username และ Password")
        ElseIf TextBox1.Text <> "" And TextBox2.Text = "" Then
            MsgBox("กรุณาระบุ Password")
        End If

        Dim userCON As New SqlClient.SqlConnection()
        Dim userCOM As New SqlClient.SqlCommand()
        Dim strConnstring, strSql As String

        strConnString = "Server=Pratcharat-pc\SQL2005;UID=sa;PASSWORD=;database=Payment;Max Pool Size=400;Connect Timeout=600;"
        userCON = New System.Data.SqlClient.SqlConnection(strConnstring)

        userCON.Open()

        strSql = "Select * from Username where Username = '" & TextBox1.Text & "' AND Password = '" & TextBox2.Text & "' "


        userCOM = New System.Data.SqlClient.SqlCommand()
        With userCOM
            .Connection = userCON
            .CommandType = CommandType.Text
            .CommandText = strSql
        End With
        userCOM.ExecuteNonQuery()
        userCON.Close()

Dim useCON As New SqlConnection()
        Dim useCom As New SqlCommand()
        Dim strConnString, strSQL As String
        strConnString = "Server=Pratcharat-pc\SQL2005;UID=sa;PASSWORD=;database=Payment;Max Pool Size=400;Connect Timeout=600;"
        useCON = New System.Data.SqlClient.SqlConnection(strConnString)
        useCON.Open()



        strSQL = "INSERT INTO Username(Username,Password,Name,Surname) " & _
        "VALUES ( '" & TextBox1.Text & "','" & TextBox2.Text & "','" & TextBox3.Text & "','" & TextBox4.Text & "' )"



        useCom = New System.Data.SqlClient.SqlCommand()
        With useCom
            .Connection = useCON
            .CommandType = CommandType.Text
            .CommandText = strSQL
        End With
        useCom.ExecuteNonQuery()



        useCom = Nothing
        useCON.Close()
        useCON = Nothing

        Me.Hide()
        Login_Form.Show()