If the user wants to group the records on particular field values then rollup is best way to do that. Rollup is a multi-stage transform function and it contains the following mandatory functions.

1. initialise

2. rollup

3. finalise

Also need to declare one temporary variable if you want to get counts of a particular group.

For each of the group, first it does call the initialise function once, followed by rollup function calls for each of the records in the group and finally calls the finalise function once at the end of last rollup call.