In the following example, how can I avoid record filtering (discarded from query results) when InvHead.SalesNo or InvHead.CourNo are left blanked ?

I have the following tables:

InvHead: (InvNo, InvDate, CustNo, SalesNo, CourNo)
InvDetl: (InvNo, Item,Qty,Price,Disc)
Customer: (CustNo,Name,Address,Phone)
Salesmen: (SalesNo,Name,Address,Phone)
Courier: (CourNo, Name Address, Phone)

My SQL stament is:

SELECT InvHead.InvNo, CustNo, InvDate, Item, Qty, Price, Disc, Customer.Name, Salesmen.Name, Courier.Name ;
FROM InvHead, InvDetl, Customer, Salesmen, Courier ;
INTO TABLE Temp ;
WHERE InvHead.InvNo = InvDetl.InvNo AND ;
InvHead.CustNo = Customer.CustNo AND ;
InvHead.SalesNo = Salesmen.SalesNo AND ;
InvHeadCourNo = Courier.CourNo ;
ORDER BY InvDate, CustNo, InvHead.InvNo