Join Dependency

• Join decomposition is a further generalization of Multivalued dependencies.
• If the join of R1 and R2 over C is equal to relation R, then we can say that a join dependency (JD) exists.
• 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 are lossless decompositions of R.
• A JD ⋈ {R1, R2,..., Rn} is said to hold over a relation R if R1, R2,....., Rn is a lossless-join decomposition.
• The *(A, B, C, D), (C, D) will be a JD of R if the join of join's attribute is equal to the relation R.
• Here, *(R1, R2, R3) is used to indicate that relation R1, R2, R3, and so on are a JD of R.