If you press Control Break it stops the query running. This has two different behaviours
For big queries, this is a possible disaster because you really do not know the state of your data.
You Have A Big Query When
You Get this prompt
|There isn't Enough disk space or memory to undo the data changes this action query is about to make|
So Is This A Problem ?
If this only happened when you pressed Control Break, it wouldn't matter because it is unlikely your users are going to do this. What does matter is that your user turns off their computer (in frustration because the query is taking so long) or finds some other way to interrupt the process. We have even had instances where switching to Excel triggers the Control Break behaviour. Then you have to decide, does this make my database incorrect.
I don't have a great solution for you apart from breaking up the query into smaller updates so it only effects smaller number of records. That way if the process is stopped mid stream, you know that the data is in its original state.
Another clunky way to avoid the problem is to inform your users with a message box or popup form that this query takes a long time so that they don't become impatient.
If you want a copy of the database that I used to test this, post a note to me on this form.
http://www.gr-fx.com.au/contact-us.html It has 1.6 million records in a table and 3 different ways to run queries.
Microsoft Access MVP since 2006