Description: A CDS abstract entity in ABAP Core Data Services (CDS) is defined within the Data
Definition Language (DDL) source code using the statement DEFINE ABSTRACT ENTITY in the
ABAP development tools for Eclipse (ADT).

• Abstract entities are utilized to define signatures without any implementation. In the
  ABAP RESTful application programming model, they can be employed to specify data
  structures for parameters and results associated with actions and functions.
• An abstract CDS entity can be used as a global structured type in ABAP programs.
• The CDS abstract entity cannot serve as a data source for SELECT statements.
• A CDS abstract entity does not create an instance as a database object, and client
  handling is not specified. These abstract entities are non-SQL CDS entities.
• A CDS abstract entity can be specified as the association target

Sample Example:

• Login in Eclipse >> Right Click on Core Data Servies >> New >> Data Definition
• Use Abstract Entity template.

• This abstract CDS entity can be evaluated using the class CL_DD_DDL_ANNOTATION_SERVICE.

Use case of CDS abstract entity:

• They can act as data types that possess properties extending beyond the conventional DDIC
  structures in the ABAP Dictionary. Instances of this application can be observed in ABAP RAP.
• They can serve as prototype definitions for data models without needing to be instantiated as
  data objects.

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