VBA - SQL with optional joins - vba

Question

I have a combolist where I can select multiple values, but if I leave them all blank then I want to exclude that portion from the query. Here is the current logic I'm using
If ModeCat_ID = "" Then
Set rs = conn.Execute("SELECT * FROM [ModeCat_T]")
Else
Set rs = conn.Execute("SELECT * FROM [ModeCat_T] WHERE [ModeCat_ID] = '" & ModeCat_ID & "'")
End If
My question is, is this the best way to go about this? I'm building a form that has a few more options so I'd rather not have a bunch of tested Ifs to check these form controls.
Second question is, if it's a combo box that can select multiples.. how would I set that up for an SQL IN statement? ie
IN ('1','2','3')
Thanks!
EDIT
Set rs = conn.Execute("SELECT * FROM [ModeCat_T]")

Answer 1

This is how I normally do it:
Dim strSQL As String
strSQL = "set nocount on; "
strSQL = strSQL & "select * "
strSQL = strSQL & "from [ModeCat_T] "
strSQL = strSQL & " where 1 = 1 "
'*******************************************************
'* Adding the other where restrictions as necessary
'*******************************************************
If Trim(Sheet1.Range("B7").Value2) <> vbNullString Then
strSQL = strSQL & " and [ModeCat_ID] = '" & Sheet1.Range("B7").Value2 & "' "
End If
If Trim(Sheet1.Range("B8").Value2) <> vbNullString Then
strSQL = strSQL & " and [ModeCat_Color] = '" & Sheet1.Range("B8").Value2 & "' "
End If
If Trim(Sheet1.Range("B9").Value2) <> vbNullString Then
strSQL = strSQL & " and [ModeCat_Size] = '" & Sheet1.Range("B9").Value2 & "' "
End If
Set rs = conn.Execute(strSQL)
Following the continued discussion about "Filters" where you can select possibly many different items from a ListBox the following code might be able to help with that:
Dim strFilter As String
Dim lngItem As Long
For lngItem = 0 To UserForm1.ListBox1.ListCount - 1
If UserForm1.ListBox1.Selected(lngItem) = True Then
If strFilter <> vbNullString Then strFilter = strFilter & ", "
strFilter = strFilter & "'"
strFilter = strFilter & UserForm1.ListBox1.List(lngItem, 1)
strFilter = strFilter & "'"
End If
Next lngItem
If strFilter <> vbNullString Then
strSQL = strSQL & "and [ModeCat_Form] in (" & strFilter & ") "
End If
Basically, it is checking which items from the list have been selected and puts them into a string (wrapped in ' and separated by a comma). Afterwards the code checks if anything made it into the "Filter String". If the filter is not empty then it should be used. BTW: it does not matter if the string contains one or many items. So, this is also permissible:
AND [ModeCat_Shape] in ('Oval')

Answer 2

I felt I should expand my comment the #Ralphs answer with a more maintainable solution.
As you're using a ComboBox and by the sound of it your wanting the user to be able to select multiple columns, use a ListBox instead. I would hide the reference to the field name in the first column of the ListBox by setting it's column width to 0. Then in the next column, you could add the actual filter in there.
Once this is done, you can iterate through the selected items to apply the filters like so:
For i = 0 To ListBox1.ListCount - 1
If ListBox1.Selected(i) Then
strSQL = " AND " & ListBox1.Column(0, i) & " = '" & ListBox1.Column(1, i) & "'"
End If
Next i