vb.net Listboxes and sql - sql

Question

I have 4 List boxes that I check,the contents of which go to make up the variables that get placed into the sql statement.This works fine.The problem is I have to check ALL of the boxes.If I leave any of the boxes out the sql statement doesn't work. I Did try adding "" to the listbox but this didn't work and it looked messy.Is there a way around this.Many Thank Jim
HERE IS MY CODE:
Imports System.Data
Imports System.Data.OleDb
Imports System.Data.Odbc
Imports System.Data.DataTable
Public Class Form1
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
End Sub
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim con As New OleDb.OleDbConnection
Dim dbProvider As String
Dim dbSource As String
Dim ds As New DataSet
Dim tables As DataTableCollection = ds.Tables
Dim source1 As New BindingSource()
Dim da As New OleDb.OleDbDataAdapter
Dim sql As String
Dim aa As String = authorList.Text
Dim bb As String = publisherList.Text
Dim cc As String = yearpublishedList.Text
Dim dd As String = genreList.Text
dbProvider = "PROVIDER=Microsoft.ACE.OLEDB.12.0;"
dbSource = "Data Source =C:\Documents and Settings\james\Desktop\Authors.accdb"
con.ConnectionString = dbProvider & dbSource
con.Open()
sql = "SELECT * FROM books WHERE author = '" & aa & "' AND publisher = '" & bb & "' AND yearpublished = '" & cc & "' AND genre = '" & dd & "' "
da = New OleDb.OleDbDataAdapter(sql, con)
da.Fill(ds, "Authors")
Dim view1 As New DataView(tables(0))
source1.DataSource = view1
DataGridView1.DataSource = view1
DataGridView1.Refresh()
'MsgBox("OPEN FOR LUNCH")
'MsgBox("CLOSED FOR THE DAY")
con.Close()
End Sub
End Class

Answer 1

You are parsing the checkboxes into a string, so when you do that, you get:
1,2,3,4,5,6,7
If you do not select one checkbox, it will look like this:
1,2,3,,5,6,7
So SQL will break
Several ways you can approach this to solve the issue, so I'm not going to attempt one. But this should hopefully explain to you what happens and why it happens so you can go ahead and fix