What is cardinality in relationships?

Parthiban Kannan

Updated on February 13, 2026

< 1 min read

Cardinality defines how many records from one table relate to another table.
Common types are one-to-one, one-to-many, and many-to-many.
In BI models we mostly use one-to-many from dimension to fact table.
For example, one customer can have many sales transactions.
Correct cardinality ensures filters flow properly in dashboards.
In one project, wrong many-to-many caused duplicate revenue totals.
After fixing to one-to-many using unique keys, numbers matched finance data.
It also impacts performance and aggregation accuracy.
So defining cardinality correctly prevents incorrect insights.