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    Thanks for this suggestion. Maybe I'm missing something, but I need to enforce "exactly one per group", not "at most one per group"... Commented Jan 28, 2015 at 19:16
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    I've edited the answer. Commented Jan 28, 2015 at 19:26
  • Thanks again, and sorry for being such a pest. After studying your answer I realized that my post was not sufficiently clear on one point. I have edited my post to remedy this. In particular, I think that the NOT NULL clause in the principal_xid INTEGER NOT NULL spec does not hold in this case. I apologize for the confusion. Commented Jan 28, 2015 at 20:36
  • @kjo No worries, I was afraid that that might be the case. Editing the answer to fix. Commented Jan 28, 2015 at 20:39
  • @ypercube: Your design is good. In the second scenario, inserting in x and y_x can be a bit challenging with the circular references. Data-modifying CTEs may be the best solution. Commented Jan 28, 2015 at 22:55