some additional explanation from F1 help on 'KEY'-statement in CDS-view:
KEY is used to define the current element as the key element of the current CDS entity. Any elements of a SELECT list can be defined as key elements. The key elements of the CDS entity are used to document the semantics of the data model. The addition key is ignored when the CDS view is activated and when accesses are performed in program executions.
More specifically, the key elements defined using KEY are ignored by the key of the CDS database view. Like in classic views, the key fields of the database view are derived implicitly from the key fields of the basis tables and the join conditions. If this is not possible, all fields of the database view are key fields.