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Editorial Team
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Asked: 2026-05-31T22:27:09+00:00

I have a class A and a class B that inherits from A. Class

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I have a class A and a class B that inherits from A. Class A has a certain attribute X. Class B adds another attribute Y. An instance of B is uniquely identified by its pair of attributes {X,Y}. So, {X,Y} is a candidate key for class B. Obviously this key doesn’t exist at the class A level since attribute Y isn’t there. I use a surrogate key for class A.

Now, I use Hibernate for the object-relational mapping. If I use the TABLE PER SUBCLASS inheritance strategy, I don’t see a possibility for defining a UNIQUE constraint on multiple attributes that belong to different classes (and then in the DB, to different tables).

My question is how can I define a unique constraint for this candidate key using Hibernate and more generally in SQL? If it’s not possible, what do you recommend?

Thanks

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1 Answer

  1. Editorial Team

    Here are two solutions:

    • Create a new table with one column which is its PK. This table should hold all the values which you want to make unique. Create foreign keys to this table which will ensure that the domain on the columns comes from the unique table.
    • Create triggers which raise an error if the condition is violated.
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