A One-to-One relationship is just a special case of One-to-Many.
Like every One-to-Many, it has a parent entity with a cardinality of 1 and referential integrity obligations.
What makes a One-to-One different is that it may have no more than one child in the relationship.
This mus be inforced procedurally, with either RDBMS or client code, since there is no relational structure to limit the number of instances.
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