Which relationship type connects exactly one instance of an entity to exactly one instance of another entity?

Introduction / Context:
Different cardinalities capture different business rules. One-to-one (1:1) relationships are less common than 1:N but appear for security partitions, optional extensions, or performance-related vertical splits.

Given Data / Assumptions:

• Each instance on side A maps to at most one instance on side B.
• Each instance on side B maps to at most one instance on side A.
• We need the standard term for this cardinality.

Concept / Approach:
A One-to-One relationship specifies a maximum cardinality of 1 on both sides. Implementation often places a foreign key with a UNIQUE constraint (or both tables share the same primary key in an identifying 1:1).

Step-by-Step Solution:

Verification / Alternative check:
SQL DDL can enforce 1:1 using UNIQUE on the foreign key column or by sharing the same primary key between paired tables.

Why Other Options Are Wrong:
One-to-Many: One maps to many, not 1:1.
Many-to-Many: Many on both sides; requires an associative table.
Composite Relationship: Not a standard cardinality term.
Recursive Relationship: Relates an entity to itself; can be 1:N or M:N, not necessarily 1:1.

Common Pitfalls:
Using 1:1 where a subtype would be clearer; if side B always exists when A exists and adds attributes, consider a subtype pattern.

Final Answer:
One-to-One Relationship