This video is taken from an actual i75 ISC Course Video.
The CPA ISC exam expects you to know relationships between tables in a database and be able to describe them as one-to-one, one-to-many, or many-to-many. The exam will also require you to know what a bridge table is, the purpose of a bridge table, and when a bridge table is needed.
A one-to-one (1:1) relationship in a database means each record in Table A is linked to only one record in Table B, and each record in Table B is linked to only one record in Table A.
Core Idea
It is a one-to-exactly-one pairing between two tables.
Simple Example (Accounting Context)
Suppose a company keeps an Employee table with EmployeeID, Name, and HireDate, and an EmployeeBenefits table with EmployeeID, HealthPlan, and RetirementPlan.
If each employee has exactly one benefits record, and each benefits record belongs to exactly one employee, that is a one-to-one relationship.
How It Is Implemented
This is usually implemented by making the primary key of one table also a foreign key in the other table, or by having both tables share the same unique ID.
Example: Employee.EmployeeID is the primary key, and EmployeeBenefits.EmployeeID is both the primary key and foreign key. This enforces that no employee can have multiple benefits records and no benefits record can belong to multiple employees.
Why Use a 1:1 Relationship?
Common reasons include data separation for security (for example payroll data vs. general HR data), logical organization, and performance or access control.
CPA-Style Takeaway
A 1:1 relationship is used when each entity instance corresponds to exactly one instance in another table, duplication and ambiguity must be avoided, and sensitive data may require restricted access.