Join Dependency in Relational Database

Join Dependency :

A join dependency is a constraint on the set of legal relations over a database scheme.  A table R is subject to a join dependency if R can always be recreated by joining multiple tables each having a subset of the attributes of R. Another way to describe a join dependency is to say that the set of relationships in the join dependency is independent of each other.

 Let R be a relation. Let R1, R2, R3 ......, Rn be arbitrary subsets of R's attributes. R satisfies the JD (Join Dependency)

        *(R1, R2, R3...., Rn)

 if and only if R is equal to the join of its projection on R1, R2, R3....., Rn

 If one of the tables in the join has all the attributes of the table R, the join dependency is called trivial. A join dependency JD(R1, R2, …, Rn) specified on relation schema R, is a trivial JD, if one of the relation schema Ri in JD(R1, R2, ….,Rn) is equal to R. The join dependency plays an important role in the Fifth normal form, also known as project-join normal form.

Next Topic : Types Of Normal Forms : 1NF, 2NF, 3NF, BCNF, 4NF and 5NF

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