About
A one-to-one relationship is a relational relationship that is equivalent to the set bijection relationship
This relationship describe that one row from a relation point to exactly one row to the other relation.
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Modeling
There are severals way to model a one-to-one relationship:
- either the two tables share the same primary keys values,
- a foreign key (FK) is held by one of the relations. This FK column should be constrained unique to simulate one-to-one multiplicity),
- or an association table is used to store the link between the 2 tables. A unique constraint has to be defined on each foreign key to ensure the one to one multiplicity.
As a relation itself is a one-to-one relational structure, it means that the two relation (tables) could also be merge in one.