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I don't know how much you know about the database, but null means empty. Besides, your study is too rigid, and it is not necessarily empty without data in it.

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What you want to query should be all overdue books, which should include two parts: borrowing and renewing overdue books:

Logical expressions should be ((status = unpaid) and (lending days > borrowing period)) or ((status = extension) and (extension days > borrowing period)).

In the front, the book is overdue, and in the back, it is overdue.

The following is the expression after where I wrote:

Where ((borrow books. Book status) = "Not returned") and ((Date ()-[Book Borrowing]! [borrowing date]) & gt=[ book category]! [lending period]) or ((book lending). Book status) = "renewal") and ((date ()-[book borrowing]! [Renewal Date]) > = [Book Category]! [loan period]);

I hope it helps you.