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Asked: 2026-05-13T18:53:33+00:00

can it make sense for one table to have multiple foreign keys? Suppose I

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can it make sense for one table to have multiple foreign keys?

Suppose I have three tables, Table A, Table B and Table C. If I think of the tables as objects (and they are mapped to objects in my code), then both Tables A and B have a many to one relationship with Table C. I.e. Table/object A and B can each have many instances of C. So the way I designed it is that Table C points to the primary key in both Table A and Table B: in other words, Table C has 2 foreign keys (a_Id, and b_Id).

Edit: I forgot to mention also that Table A can have many instances of Table B. So Table B has a foreign key into Table A. If this makes a difference…

I am wondering if this makes sense or is there a better way to do it? Thanks.

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  1. Editorial Team

    This is fine, but note that it only makes sense if a C always has to have both an A and a B as a pair.

    If you just want A’s to have C’s and B’s to have C’s, but A and B are otherwise unrelated then you should put the foreign key in A and in B and allow it to be nullable.

    Update: after clarification it seems you want two separate relationships: an A can have many Cs, and a B can have many Cs, but a C can only belong to one A or one B.

    Solution: It’s two separate one-to-many relationships, so create two new tables A_C and B_C, and put the foreign keys there. A_C contains a foreign key to A and a foreign key to C. Similarly for B_C.

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