how do i put an sql statement inside if-else statement?? or is that possible?
my aim is to check if the listbox is not selected(options!=0) then I will add sql statement ("AND table.field=value")

my code:
<%
sql = "SELECT * FROM ..WHERE table.field ='"& value & "'"

if (document.formname.listboxname.options[0].value!=0)
    sqlsort = "AND table1.field1 ='"& value1 & "'"
end if
%>

Assuming that the code is not on the page that the form is on, but rather on another page that the form submits to, you can try this. Let me know if it's what you're looking for.

<%
    ListBox = request("ListBox")

    set rs = server.createobject("adodb.recordset")
    sql = "select * from table"
    sql = sql & " where field1 = '" & Value1 & "'"
    if ListBox = "" then
        sql = sql & " and field2 = '" & Value2 & "'"
    end if
    rs.open sql, db, 3, 3
%>