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Data & Analytics · Intermediate

One-to-One Relationship (1:1)

A relationship where each row in one table matches at most one row in another.

Definition

1:1 relationships are often used to split rarely used or sensitive columns into a separate table, or to model subtypes. They are implemented with a foreign key that is also unique, often the shared primary key.

Example

Employee and EmployeeSecurityDetails sharing EmployeeID as their primary key.

Exam tip

In Power BI, a 1:1 relationship often means the two tables should simply be merged.

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