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Editorial Team
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Asked: 2026-05-16T20:22:59+00:00

Suppose I have a table Books with columns Book_ID (Primary Key), ISBN (Unique), and

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Suppose I have a table Books with columns Book_ID (Primary Key), ISBN (Unique), and several others such as Title/author/etc.

I have another table Sales, with primary key Sale_ID, a foreign key to link to Books, and other fields with info on Sales. Sales only exist for books with ISBNs. Is it better database design to have Book_ID or ISBN be the primary key. Note: I will be LEFT JOINing Sales to Books on whichever the foreign key is chosen.

UPDATE: Some Books have no ISBN’s because they’re not published with them. However, I don’t foresee (at least in the next several years) users being able to sell them, because I have no system for making sure that a given book w/o the ISBN isn’t repeated.

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1 Answer

  1. Editorial Team

    In general, I think I would go for the surrogate primary key book_id as the foreign key.

    There are a few potential issues that I can identify if you were to use the non-primary key ISBN as a foreign key:

    • You might have a missing or unknown ISBN for a particular book. What would you do in case there is a sale of that book?
    • You might have an incorrect ISBN. To edit it, you would have to update all the tables that would be using it as a foreign key.
    • You might want to start selling books that don’t have an ISBN in the future. Why are you storing books without an ISBN in the first place?
    • An index on book_id should be much more compact than one on an ISBN field.
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