In addition to CLUSTER and POOLED tables ABAP forbids joins with projection views.
This type of data is also defined in DD02L, as tabclass = 'VIEW' and VIEWCLASS = 'P'.
Projection views are used to hide fields of a table. This can minimize interfaces; for example when you access the database, you only read and write the field contents actually needed.
A projection view contains exactly one table. You cannot define selection conditions for projection views.
There is no corresponding object in the database for a projection view. The R/3 System maps the access to a projection view to the corresponding access to its base table. You can also access pooled tables and cluster tables with a projection view.
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