In database design, what does a one-to-one relationship mean? Choose the option that best describes a case where one record of a given record type is related to only one record of another record type (and vice versa), as used in entity–relationship modeling and relational schema design.

Introduction / Context:
Understanding cardinality is foundational in database design. A one-to-one relationship is one of the three classic relationship types (one-to-one, one-to-many, many-to-many) used in entity–relationship (ER) modeling and relational schema design. This question checks your grasp of what “one-to-one” implies both logically and in practical implementation.

Given Data / Assumptions:

• We have two record types (entities) with a relationship between them.
• Each record of the first type maps to at most one record of the second type.
• Each record of the second type maps to at most one record of the first type.

Concept / Approach:
In a one-to-one relationship, each entity occurrence corresponds to zero or one occurrence in the related entity and vice versa. Designers often use this pattern to split rarely used or sensitive columns into a separate table, or to model optional extensions of an entity where duplication would be wasteful or a single wide table would be inflexible.

Step-by-Step Solution:
Identify the definition: “one record … related to only one record …” matches one-to-one.Contrast with one-to-many: one parent can have many children.Contrast with many-to-many: many on both sides, typically resolved via a junction table.Confirm cardinality: each side has maximum cardinality of one.

Verification / Alternative check:
Implementation frequently uses a shared primary key or a unique foreign key to enforce one-to-one. If a foreign key in table B references table A’s primary key and is constrained UNIQUE, each row in A can be referenced by at most one row in B, enforcing one-to-one.

Why Other Options Are Wrong:

• One-to-many / Many-to-one: Allow multiple related records on one side.
• Many-to-many: Requires an associative (junction) table; not one-to-one.
• None of the above: The exact definition is covered by “One-to-one relationship.”

Common Pitfalls:
Confusing “optional” with “one-to-one”: optionality is about minimum cardinality, not maximum. Also, assuming one-to-one must always be merged into a single table; normalization and access patterns can justify separation.

Final Answer:
One-to-one relationship