Notion’s ability to relate databases is among its most useful features, and the
Rollup property is a robust extension of
Relations. By retrieving property values from related items, you can automatically populate content and summarize information. For example, you can automatically categorize people based on their employers; you can total expenses by month, year, category and other classifications; and you can calculate the progress of a project as the percentage of its completed tasks.
The Notion VIP post Build Smarter Databases with the Rollup Property expands on these examples to demonstrate the vast utility of the
Rollup property, and this template includes the databases described.