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Asked: 2026-06-13T22:56:07+00:00

I couldn’t figure out what terms to google, so help tagging this question or

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I couldn’t figure out what terms to google, so help tagging this question or just pointing me in the way of a related question would be helpful.

I believe that I have a typical many-to-many relationship:

CREATE TABLE groups (
  id integer PRIMARY KEY);

CREATE TABLE elements (
  id integer PRIMARY KEY);

CREATE TABLE groups_elements (
  groups_id integer REFERENCES groups,
  elements_id integer REFERENCES elements,
  PRIMARY KEY (groups_id, elements_id));

I want to have a constraint that there can only be one groups_id for a given set of elements_ids.

For example, the following is valid:

groups_id | elements_id
        1 | 1
        1 | 2
        2 | 2
        2 | 3

The following is not valid, because then groups 1 and 2 would be equivalent.

groups_id | elements_id
        1 | 1
        1 | 2
        2 | 2
        2 | 1

Not every subset of elements must have a group (this is not the power set), but new subsets may be formed. I suspect that my design is incorrect since I’m really talking about adding a group as a single entity.

How can I create identifiers for subsets of elements without risk of duplicating subsets?

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1 Answer

  1. Editorial Team

    This link from user ypercube was most helpful: unique constraint on a set. In short, a bit of what everyone is saying is correct.

    It’s a question of tradeoffs, but here are the best options:

    a) Add a hash or some other combination of element values to the groups table and make it unique, then populate the groups_elements table off of it using triggers. Pros of this method are that it preserves querying ability and enforces the constraint so long as you deny naked updates to groups_elements. Cons are that it adds complexity and you’ve now introduced logic like “how do you uniquely represent a set of elements” into your database.

    b) Leave the tables as-is and control the access to groups_elements with your access layer, be it a stored procedure or otherwise. This has the advantage of preserving querying ability and keeps the database itself simple. However, it means that you are moving an analytic constraint into your access layer, which necessarily means that your access layer will need to be more complex. Another point is that it separates what the data should be from the data itself, which has both pros and cons. If you need faster access to whether or not a set already exists, you can attack that problem separately.

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