Query returning all values if combo box null

I am currently using a query that checks values selected in 3 separate combo boxes with the fields in the table, if they match up, the query returns matching results (where all 3 fields line up).

However, I wish to change the queries such that if a field is left blank in the combo box on the form, for that part of the query, all results are returned. eg. if there is no user value in the combo box, all users are returned subject to the alignment in the other two queries.

The 3 fields that are currently used in the query are:

1) User (User_check in the combo box)
2) Product (product in the combo box)
3) Trade Type (TradeType in the combo box)

Here is the code in SQL as it stands:

SELECT tblBusted.Date, tblBusted.Time, tblBusted.DealId, tblBusted.TradeType, tblSupervisors.User, tblBusted.Product, tblBusted.Date, tblBusted.Date
FROM tblSupervisors INNER JOIN tblBusted ON tblSupervisors.User = tblBusted.User
GROUP BY tblBusted.Time, tblBusted.DealId, tblBusted.TradeType, tblSupervisors.User, tblBusted.Product, tblBusted.Date, tblBusted.MarketId, tblBusted.Strip, tblBusted.BuyerName, tblBusted.BuyerCompany, tblBusted.SellerName, tblBusted.SellerCompany, tblBusted.Date
HAVING (((tblBusted.TradeType)=Forms!frmQueryState!TradeType) And ((tblSupervisors.User)=Forms!frmQueryState!User_check) And ((tblBusted.Product)=Forms!frmQueryState!product) And ((tblBusted.Date) Between Forms!frmQueryState!Fromdate And Forms!frmQueryState!Todate));


Any help would be appreciated.

Regards,

Frederick