Rollup Column Data Types Into Dynamics 365 CRM/Model Driven App

Rollup column behavior are used to perform the operation on related tables. This operation occur from Source table to Related table.

Using it you can perform the following below operations.
1. SUM
2. MAX
3. MIN
4. COUNT
5. AVG

Open you solution => Go to table (Parent table Country) => Create New Column => Behavior (Rollup) => Save.
Here I am counting the related child table(Student Registration) total records.

Once you save the column one new window will be open. Here you have to select the Source table, Related table, Filter and Aggregation.
Here I am Counting the Child table (Student Registration) total records.