Explain the project-join normal form, Database Management System

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Explain the Project-Join normal form

A relation R is in PJNF along with respect to a set of functional dependencies D if for all join dependencies of the outline *(R1, R2, ...,Rn) and R =  R1 U R2 U ..., URn at least one of the subsequent holds:

 *(R1, R2, ...,Rn) is trivial

 Each Ri is a superkey of R


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