CDS table function + AMDP

When CDS cannot express the logic — a recursive CTE, or a window function such as `row_number()` for ranking before CDS supported it — drop one rung to a CDS table function whose body is an AMDP method. The table function declares the returned structure and parameters in DDIC-like syntax; the AMDP method supplies the SQLScript that produces those rows. The function is then consumable from Open SQL and other CDS just like a view.

The AMDP read method is implemented `by database function for hdb language sqlscript options read-only using ...`. `options read-only` tells HANA the method does not write, which lets it route and optimize the call on scale-out systems; the `using` clause must name every database object the SQLScript touches so the dependency analyzer wires activation correctly. The body returns a SELECT — here with `row_number() over ( partition by ... order by ... )` to rank rows per customer.

The non-negotiable rule is client handling: AMDP has *no implicit MANDT injection* the way Open SQL does. You must filter `where mandt = :p_client` yourself, where `p_client` is a parameter annotated `@Environment.systemField: #CLIENT` so the framework fills it with the caller's client. Forget it and the method returns cross-client data — a defect ATC's AMDP checks catch, but only when those checks are enabled in your variant.

ABAP@EndUserText.label: 'Order Ranking'
define table function ZI_OrderRanking
  with parameters
    @Environment.systemField: #CLIENT
    p_client : abap.clnt
  returns {
    client      : abap.clnt;
    OrderId     : zorder_id;
    CustomerId  : kunnr;
    RankPerCust : abap.int4;
  }
  implemented by method zcl_order_ranking=>get;

ABAPclass zcl_order_ranking definition public.
  public section.
    class-methods get for table function zi_orderranking.
endclass.

class zcl_order_ranking implementation.
  method get by database function for hdb
             language sqlscript
             options read-only
             using zorder_hdr.
    return
      select
        mandt    as client,
        order_id as "OrderId",
        kunnr    as "CustomerId",
        row_number() over ( partition by kunnr order by total desc )
                 as "RankPerCust"
      from zorder_hdr
      where mandt = :p_client;
  endmethod.
endclass.

Source notes: clean-core-curriculum §6.3