CDS patterns that compress code

A surprising amount of classic ABAP — nested loops, COLLECT statements, manual subtotalling — collapses into a few lines of CDS. The three patterns that compress the most code are aggregation (count, sum, max, min, avg), associations (declared joins you navigate by name), and group by. Written as a `define view entity ... as select from ...`, the view is pushed to HANA and consumable everywhere.

Aggregation replaces hand-rolled totalling: `count(*)`, `sum(h.total)`, `max(h.orderdate)` are computed on the database in a single scan. Associations replace repeated explicit joins: you declare `association [0..*] to zorder_item as _items on ...` once and expose or navigate it by name, and the join is materialized only when the association is actually used. Together they turn a screen of procedural code into a declarative contract.

The rule that commonly trips up newcomers: when a view aggregates, the `group by` must list *every* non-aggregated field in the select list. Miss one and the view often activates but errors at runtime — the activator catches many cases, but not when an expression appears in the select. Treat it as a discipline: every plain field in the projection appears verbatim in the group by.

ABAPselect customerid, total, orderdate
  from zorder_hdr
  where status = 'OPEN'
  into table @data(lt_hdr).
loop at lt_hdr assigning field-symbol(<h>).
  ls_sum-customerid = <h>-customerid.
  ls_sum-ordercount = 1.
  ls_sum-totalopen  = <h>-total.
  collect ls_sum into lt_sum.
endloop.

ABAP@AccessControl.authorizationCheck: #NOT_REQUIRED
@EndUserText.label: 'Open Orders By Customer'
define view entity ZI_OpenOrdersByCustomer
  as select from zorder_hdr as h
  association [0..*] to zorder_item as _items
    on _items.OrderId = h.OrderId
{
  key h.CustomerId,
      count(*)         as OrderCount,
      sum(h.Total)     as TotalOpenAmount,
      max(h.OrderDate) as LastOrderDate,
      _items
}
where h.Status = 'OPEN'
group by h.CustomerId

Source notes: clean-core-curriculum §6.2