Lesson 3 Companion

This companion expands Lesson 3 — CDS Expressions, Calculated Elements, and Filtering.

It contains deeper explanations, complete reference implementations, practice solutions, troubleshooting guidance, and current SAP references.

How to use this companion

Use the main lesson to build the artifacts first. Use this companion to review syntax, compare implementations, or investigate why the data model is structured in a particular way.

Related Lesson 3 files:

1. Lesson 3 — CDS Expressions, Calculated Elements, and Filtering
2. Lesson 3 Practice — Connection Modeling

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1. Why Lesson 3 uses /DMO/FLIGHT

/DMO/FLIGHT provides a simple reusable calculation:

SeatsAvailable = seats_max - seats_occupied

This demonstrates arithmetic CDS expressions without requiring joins, associations, aggregates, currency conversions, or transactional behavior.

SAP's ABAP Flight Reference Scenario documents /DMO/FLIGHT with fields including:

carrier_id
connection_id
flight_date
price
currency_code
plane_type_id
seats_max
seats_occupied

The key fields are:

carrier_id
connection_id
flight_date

---

2. Current CDS View Entity expression scope

Current SAP documentation lists expression families for CDS View Entities including:

arithmetic expressions
aggregate expressions
CASE expressions
CAST expressions
built-in functions

View Entities also support clauses such as WHERE, GROUP BY, and HAVING, plus relationship modeling with joins and associations.

Lesson 3 uses a focused subset.

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3. Arithmetic calculated elements

The lesson uses:

Flight.seats_max - Flight.seats_occupied as SeatsAvailable

This is evaluated per result row.

The expression does not alter persistence.

Persisted:
  seats_max
  seats_occupied

Calculated CDS element:
  SeatsAvailable

---

4. Why not occupancy percentage yet?

A tempting calculation is:

SeatsOccupied / SeatsMax * 100

The lesson avoids it because division introduces:

result-type decisions
division-by-zero handling
integer vs decimal behavior
rounding
CAST / DIVISION considerations

Those are useful topics, but they would distract from the first expression lesson.

---

5. CASE expression

The lesson uses a searched CASE:

case
  when Flight.seats_occupied = Flight.seats_max
    then abap.char'Full'
  else abap.char'Available'
end as AvailabilityStatus

Current CDS View Entity syntax supports both simple and searched CASE forms.

The important distinction is:

CASE  → creates a value
WHERE → restricts rows

---

6. Typed literals

Current SAP keyword documentation recommends typed literals where applicable.

Syntax:

abap.dtype'value'

Examples:

abap.char'Available'
abap.int4'100'
abap.tims'120000'
abap.dats'20261231'

Typed literals make the intended Dictionary type explicit and improve compatibility checking.

---

7. Typed time literals

The practice assignment uses:

abap.tims'120000'
abap.tims'180000'

abap.tims uses the HHMMSS representation.

Examples:

120000 = 12:00:00
180000 = 18:00:00

Do not write:

12:00
12:00:00

inside an abap.tims literal.

---

8. Model layering

The lesson creates:

/DMO/FLIGHT
      ↓
ZI_FLIGHT_###
      ↓
ZI_AVAILABLE_FLIGHT_###

The base entity calculates:

SeatsAvailable
AvailabilityStatus

The specialized entity reuses those elements and filters:

AvailabilityStatus = Available

This avoids repeating the CASE expression.

---

9. Why a second entity instead of repeating the expression?

Avoid:

Base CDS calculates status
Specialized CDS repeats CASE
Reader repeats CASE

Prefer:

Base CDS calculates once
      ↓
Specialized CDS reuses
      ↓
Reader consumes

That is a core benefit of a reusable data model.

---

10. Model-level vs consumer-level filtering

Consumer filter:

SELECT FROM zi_flight_###
  ...
  WHERE AvailabilityStatus = 'Available'

This restriction belongs to one request.

Specialized model:

ZI_AVAILABLE_FLIGHT_###

This entity itself represents available flights.

Choose the layer based on the meaning of the requirement.

---

11. CDS condition capabilities

Current CDS conditions support comparison operators including:

=
<>
<
>
<=
>=

and Boolean composition with:

NOT
AND
OR

Other supported condition forms include BETWEEN, LIKE, IS NULL, and IS INITIAL in appropriate CDS conditions.

---

12. SQL evaluation-order caution

SAP documents that SQL does not guarantee short-circuit evaluation order for conditions combined with AND.

Therefore, do not rely on logic such as:

x <> 0 AND y / x > 1

assuming the second expression can never be evaluated when x = 0.

For potentially unsafe expressions, a CASE expression can be a safer design.

This detail is background knowledge rather than a required Lesson 3 exercise.

---

13. CDS entities as ABAP types

An activated CDS View Entity is available in ABAP as a global structured type.

Therefore:

TYPES tt_flights TYPE STANDARD TABLE OF zi_flight_###
  WITH EMPTY KEY.

is valid conceptually for a Reader that deliberately exposes the CDS model as its contract.

This differs from the earlier Carrier Reader, which preserved a manually defined Reader structure.

Both designs are useful:

Independent Reader contract
or
CDS model as Reader contract

Lesson 3 demonstrates the second.

---

14. Why FIELDS * is acceptable here

The method declares that it returns the complete CDS entity type.

Therefore:

SELECT FROM zi_flight_###
  FIELDS *

matches the method contract.

This is different from casually using SELECT * against a large persistence table.

---

15. * in CDS vs ABAP SQL

CDS View Entity element list:

{
  *
}

is not supported for exposing all source elements.

ABAP SQL:

FIELDS *

is valid.

Do not treat CDS DDL and ABAP SQL syntax as identical.

---

16. Base Flight CDS reference

@AccessControl.authorizationCheck: #NOT_REQUIRED
@EndUserText.label: 'Flight'

define view entity ZI_Flight_###
  as select from /dmo/flight as Flight
{
  key Flight.carrier_id      as CarrierId,
  key Flight.connection_id   as ConnectionId,
  key Flight.flight_date     as FlightDate,
      Flight.plane_type_id   as PlaneTypeId,
      Flight.seats_max       as SeatsMax,
      Flight.seats_occupied  as SeatsOccupied,
      Flight.seats_max - Flight.seats_occupied
                               as SeatsAvailable,
      abap.char'Flight'       as RecordType,

      case
        when Flight.seats_occupied = Flight.seats_max
          then abap.char'Full'
        else abap.char'Available'
      end                      as AvailabilityStatus
}

---

17. Filtered Flight CDS reference

@AccessControl.authorizationCheck: #NOT_REQUIRED
@EndUserText.label: 'Available Flights'

define view entity ZI_Available_Flight_###
  as select from ZI_Flight_### as Flight
{
  key Flight.CarrierId,
  key Flight.ConnectionId,
  key Flight.FlightDate,
      Flight.PlaneTypeId,
      Flight.SeatsMax,
      Flight.SeatsOccupied,
      Flight.SeatsAvailable,
      Flight.RecordType,
      Flight.AvailabilityStatus
}
where Flight.AvailabilityStatus = abap.char'Available'

---

18. Flight Reader reference

CLASS zcl_flight_reader_### DEFINITION
  PUBLIC
  FINAL
  CREATE PUBLIC.

  PUBLIC SECTION.

    TYPES tt_flights TYPE STANDARD TABLE OF zi_flight_###
      WITH EMPTY KEY.

    TYPES tt_available_flights
      TYPE STANDARD TABLE OF zi_available_flight_###
      WITH EMPTY KEY.

    METHODS get_all
      RETURNING VALUE(flights) TYPE tt_flights.

    METHODS get_available
      RETURNING VALUE(flights) TYPE tt_available_flights.

ENDCLASS.


CLASS zcl_flight_reader_### IMPLEMENTATION.

  METHOD get_all.

    SELECT FROM zi_flight_###
      FIELDS *
      ORDER BY CarrierId,
               ConnectionId,
               FlightDate
      INTO TABLE @flights.

  ENDMETHOD.


  METHOD get_available.

    SELECT FROM zi_available_flight_###
      FIELDS *
      ORDER BY CarrierId,
               ConnectionId,
               FlightDate
      INTO TABLE @flights.

  ENDMETHOD.

ENDCLASS.

---

19. Runner reference

CLASS zcl_cds_l03_### DEFINITION
  PUBLIC
  FINAL
  CREATE PUBLIC.

  PUBLIC SECTION.
    INTERFACES if_oo_adt_classrun.

ENDCLASS.


CLASS zcl_cds_l03_### IMPLEMENTATION.

  METHOD if_oo_adt_classrun~main.

    DATA(reader) = NEW zcl_flight_reader_###( ).

    DATA(all_flights) = reader->get_all( ).
    out->write( all_flights ).

    DATA(available_flights) = reader->get_available( ).
    out->write( available_flights ).

  ENDMETHOD.

ENDCLASS.

---

20. Practice source: /DMO/CONNECTION

SAP documents /DMO/CONNECTION with:

carrier_id          key
connection_id       key
airport_from_id
airport_to_id
departure_time
arrival_time
distance
distance_unit

The practice uses this source for:

CONCAT_WITH_SPACE
CASE
typed time literals
filtered CDS layering

without introducing joins.

---

21. Practice base entity reference

@AccessControl.authorizationCheck: #NOT_REQUIRED
@EndUserText.label: 'Connection'

define view entity ZI_Connection_###
  as select from /dmo/connection as Connection
{
  key Connection.carrier_id       as CarrierId,
  key Connection.connection_id    as ConnectionId,
      Connection.airport_from_id  as AirportFromId,
      Connection.airport_to_id    as AirportToId,
      Connection.departure_time   as DepartureTime,
      Connection.arrival_time     as ArrivalTime,
      Connection.distance         as Distance,
      Connection.distance_unit    as DistanceUnit,

      concat_with_space(
        Connection.airport_from_id,
        Connection.airport_to_id,
        1
      )                            as RouteCode,

      case
        when Connection.departure_time < abap.tims'120000'
          then abap.char'Morning'
        when Connection.departure_time < abap.tims'180000'
          then abap.char'Afternoon'
        else abap.char'Evening'
      end                          as DeparturePeriod
}

---

22. Why CONCAT_WITH_SPACE

Current View Entity function scope includes string functions such as:

CONCAT
CONCAT_WITH_SPACE
LENGTH
LOWER
UPPER
LEFT
RIGHT

The practice uses:

concat_with_space(
  Connection.airport_from_id,
  Connection.airport_to_id,
  1
)

to create a simple route representation such as:

FRA JFK

---

23. Practice filtered entity reference

@AccessControl.authorizationCheck: #NOT_REQUIRED
@EndUserText.label: 'Morning Connections'

define view entity ZI_Morning_Connection_###
  as select from ZI_Connection_### as Connection
{
  key Connection.CarrierId,
  key Connection.ConnectionId,
      Connection.AirportFromId,
      Connection.AirportToId,
      Connection.DepartureTime,
      Connection.ArrivalTime,
      Connection.Distance,
      Connection.DistanceUnit,
      Connection.RouteCode,
      Connection.DeparturePeriod
}
where Connection.DeparturePeriod = abap.char'Morning'

---

24. Practice Reader reference

CLASS zcl_connection_reader_### DEFINITION
  PUBLIC
  FINAL
  CREATE PUBLIC.

  PUBLIC SECTION.

    TYPES tt_connections
      TYPE STANDARD TABLE OF zi_connection_###
      WITH EMPTY KEY.

    TYPES tt_morning_connections
      TYPE STANDARD TABLE OF zi_morning_connection_###
      WITH EMPTY KEY.

    METHODS get_all
      RETURNING VALUE(connections) TYPE tt_connections.

    METHODS get_morning
      RETURNING VALUE(connections) TYPE tt_morning_connections.

ENDCLASS.


CLASS zcl_connection_reader_### IMPLEMENTATION.

  METHOD get_all.

    SELECT FROM zi_connection_###
      FIELDS *
      ORDER BY CarrierId,
               ConnectionId
      INTO TABLE @connections.

  ENDMETHOD.


  METHOD get_morning.

    SELECT FROM zi_morning_connection_###
      FIELDS *
      ORDER BY CarrierId,
               ConnectionId
      INTO TABLE @connections.

  ENDMETHOD.

ENDCLASS.

---

25. Practice Runner reference

CLASS zcl_cds_l03_practice_### DEFINITION
  PUBLIC
  FINAL
  CREATE PUBLIC.

  PUBLIC SECTION.
    INTERFACES if_oo_adt_classrun.

ENDCLASS.


CLASS zcl_cds_l03_practice_### IMPLEMENTATION.

  METHOD if_oo_adt_classrun~main.

    DATA(reader) = NEW zcl_connection_reader_###( ).

    DATA(connections) = reader->get_all( ).

    LOOP AT connections INTO DATA(connection).

      out->write(
        |{ connection-RouteCode } - { connection-DeparturePeriod }|
      ).

    ENDLOOP.

    DATA(morning_connections) = reader->get_morning( ).

    out->write( morning_connections ).

  ENDMETHOD.

ENDCLASS.

---

26. Troubleshooting

SeatsAvailable is unknown in the filtered entity

Confirm ZI_FLIGHT_### is active and exposes SeatsAvailable.

AvailabilityStatus comparison fails

Confirm the spelling and the typed literal:

abap.char'Available'

abap.tims is rejected

Check the format:

abap.tims'120000'

CONCAT_WITH_SPACE cannot resolve fields

Use source-qualified fields:

Connection.airport_from_id
Connection.airport_to_id

FIELDS * causes confusion

Remember:

CDS element list → no *
ABAP SQL         → FIELDS * is valid

---

27. Suggested pacing

Section	Approx. time
Inspect /DMO/FLIGHT	10 min
Arithmetic calculated element	15 min
CASE expression	15 min
Typed literals	10 min
Data Preview + SQL Console	15 min
Filtered CDS layer	20 min
CDS entity as ABAP type	10 min
Reader + Runner	20 min
Review	5 min

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28. Current SAP references

• CDS View Entities — Feature Scope
• CDS View Entity Expressions
• CDS View Entity CASE Expressions
• CDS View Entity Numeric Functions
• CDS View Entity Built-In Functions
• CDS Conditions
• Comparison Operators
• CDS View Entity Select List
• Typed Literals
• CDS Entities as ABAP Types
• ABAP Flight Reference Scenario