Hey. this may seem obvious but i cannot find a way to solve this problem. i have a booking system and want to create a query that only lets a Customer ID book onto a course only once, but to many others. and to also display an error message if the customer tries to book on the same course twice. wondering if anyone could help, would be very grateful
Regards
Stu

its not obvious and we would need to know your table strucutre to advise.

What you should have is a join table that lists CustomerID and CourseID. You can set a multi-field index on those two to prevent a customer being entered more than once for a course.