Database Systems: Concepts, Design and Applications

10.6. Join Dependencies and Fifth Normal Form (5NF)

The anomalies of MVDs and are eliminated by join dependency (JD) and 5NF.

10.6.1. Join Dependencies (JD)

A join dependency (JD) can be said to exist if the join of R₁ and R₂ over C is equal to relation R. Where, R₁ and R₂ are the decompositions R₁(A, B, C), and R₂ (C,D) of a given relations R (A, B, C, D). Alternatively, R₁ and R₂ is a lossless decomposition of R. In other words, *(A, B, C, D), (C, D) will be a join dependency of R if the join of the join’s attributes is equal to relation R. Here, *(R₁, R₂, R₃, ....) indicates that relations R₁, R₂, R₃ and so on are a join dependency (JD) of R. Therefore, a necessary condition for a relation R to satisfy a JD *(R₁, R₂,...., R_n) is that

R