If Statement help required

[G Marshall]

I have an input form “OilOrders” linked to a table “OrdersInvoices” Three input fields on the form are “InvoiceNumber” , “Date” and SiteName”. I want to create an event procedure on the field “SiteName” that produces a message box with a warning if there is already a duplicate “InvoiceNumber”, “Date” and SiteName” already in the table “OrderInvoices”.
Now it is possible that there could be a duplicate entry in these fields, I only want to warn the user and ask if they wish to continue.
Could someone point me in the right direction as to what code I will need.
Any help appreciated.


Gerald