Association Aggregation Composition - suniladhya/Advantage GitHub Wiki

link One-to-One relationships should be generally merged simply into one table. If there aren't any contradictions, the One-to-One relationship might be a sign of an unconsidered decision.

And If You really want to use this kind of relationship, it's totally up to You where to place FK. You might want to take optionality into consideration when applying FK. However, in MySQL, it still won't be a true One-to-One relationship because deferred keys are not supported there.