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Incoming links: Constraints in the PostgreSQL documentation
Foreign key Created 2026-10-31 Updated 2026-11-05
A foreign key is a database integrity constraint requiring designated database attributes to match a referenced identifier in a database relation. The referenced database relation may be the same one: a self-reference can represent each employee's manager. The identifier being referenced must have the required uniqueness, but the referencing database attributes need not be unique; several employees can have the same manager.[ref]
Constraints in the PostgreSQL documentation. version 18, section 5.5.5, foreign keys and self-referential tree example
For a simple example, let
Instrument(id) contain identifiers . A measurement carrying instrument_id=4 has a matching scientific instrument; one carrying instrument_id=7 violates the foreign key. Deleting scientific instrument four while measurements still refer to it requires a stated policy: reject the deletion, cascade a suitable deletion, or apply another permitted referential action.[ref]SQL NULL and compound foreign keys require an explicit matching policy. A nullable simple reference can represent no parent, whereas
NOT NULL can require a reference. A foreign key enforces reference existence; it does not by itself prove that a self-referencing structure has no directed cycles.