Flow

Rollup summary field is an interesting custom field data type in salesforce. This will work when two objects have Master-Detail relationship but not for lookup relationship. Here I am going to explain a way to have rollup for lookup relationship.

Use case:

Rollup summary fields for lookup relationship.

Solution:

Using Flow and Process Builder we can do Rollup summary fields (Sum,Count,Max,Min) on object which is parent of lookup relationship.