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CDS View activation generates 2 objects:
CDS View Entity(consumed everywhere) & SQL view at DDIC level.
Cardinality is used to define the relationship among tables. It can be such as ZERO TO ONE, ONE to ONE,ONE To MANY, MANY to ZERO represented as [0..1],[1..1], [1..*], [*..0]
Path expression is like a where condition which is used inside the CDS select to filter the data.
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The dictionary meaning for fallback is 'an alternative plan that may be used in an emergency'. That's what exactly the fallback class is meant for. Fallback class comes into picture when there is no active implementation exists for the BADI. In such case the code in fallback class's method executes. As soon as an implementation exists in system, the implementation call will be used automatically. This fallback class we give when we create the BADI definition.
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