For a one-to-one (1:1) relationship between two tables, where should the foreign key be placed to implement the relationship in the relational schema?

Introduction / Context:
Unlike one-to-many relationships, a one-to-one (1:1) relationship offers flexibility in where you place the foreign key. The design choice depends on optionality, access patterns, and normalization goals.

Given Data / Assumptions:

• Two tables are in a 1:1 relationship.
• Each row in Table A matches at most one row in Table B, and vice versa.
• You may have optional participation on one side (partial specialization).

Concept / Approach:
In a 1:1, the foreign key can reside in either table. A common approach is to place the FK in the table that is optional (the “child” in an identifying sense), possibly also making it the primary key (PK = FK) to enforce strict 1:1. If both sides are mandatory, choose the table that best fits access and lifecycle semantics.

Step-by-Step Solution:

Verification / Alternative check:
Attempt to insert multiple rows referencing the same counterpart; UNIQUE/PK=FK should prevent violations, confirming a true 1:1.

Why Other Options Are Wrong:

• Parent-only or child-only as a rule: Overly rigid; correct placement depends on design.
• Either table without defining parent/child: You still need to define constraint semantics; “either without specifying” is vague.
• Separate junction table only: Junctions are for many-to-many, not necessary for 1:1.

Common Pitfalls:
Accidentally creating a 1:N by not enforcing uniqueness on the foreign key. Always ensure FK has a UNIQUE or PK constraint to maintain 1:1.

Final Answer:
either the parent table or the child table.