Core Data Services - Aggregation functions and general questions

Hi everybody,

i have three questions concerning Core Data Services for ABAP:

1. I tried to create a CDS-View, which sums up all values for the field luggweight from the flight data model table sbook.

Here is the code:

@AbapCatalog.sqlViewName: 'zsql_view_2'

define view Zexample2_Ddl as select

  sum(luggweight) as sumlweight

  from sbook

I also tried the syntax with the brackets, but the compiler says, that the group by clause is missing. But i dont want to group my summarization.

Has anybody an idea how to achieve my requirement ?

2. How are Core Data Services internally implemented ? Are they based on ABAP ?

3. Why is it necessary to implement core data services in Abap Development Tools instead of using transaction se80 ? Does anybody know the      technical reasons ?

Kind Regards