Referential Integrity

Referential integrity is a relational model constraint that ensures every Foreign Key value in a child table either matches an existing Primary Key value in the parent table or is NULL. It prevents orphan records — rows that reference data that doesn’t exist.

Enforcement

The DBMS checks referential integrity on every:

• INSERT into child table — FK value must exist in parent
• UPDATE of FK in child — New value must exist in parent
• DELETE from parent — Must not leave orphan children (unless cascade/set null)
• UPDATE of PK in parent — Must not break existing references

Referential Actions

What happens when a referenced parent row changes. See Database Constraints for syntax:

Action	Effect on child rows
RESTRICT	Block the operation
CASCADE	Delete/update child rows to match
SET NULL	Set FK to NULL
SET DEFAULT	Set FK to default value

Violations and Orphans

Without referential integrity:

-- Parent deleted, orphan remains
DELETE FROM customers WHERE id = 1;
-- orders.customer_id = 1 still exists → orphan record
-- Joins return no customer info, aggregations are wrong

With referential integrity, the DELETE is either blocked (RESTRICT) or handled (CASCADE, SET NULL).