Join dependency

In the area of computer science known as dependency theory, a join dependency is a constraint on the set of legal relations over a database scheme. A table T {displaystyle T} is subject to a join dependency if T {displaystyle T} can always be recreated by joining multiple tables each having a subset of the attributes of T {displaystyle T} . If one of the tables in the join has all the attributes of the table T {displaystyle T} , the join dependency is called trivial. In the area of computer science known as dependency theory, a join dependency is a constraint on the set of legal relations over a database scheme. A table T {displaystyle T} is subject to a join dependency if T {displaystyle T} can always be recreated by joining multiple tables each having a subset of the attributes of T {displaystyle T} . If one of the tables in the join has all the attributes of the table T {displaystyle T} , the join dependency is called trivial. The join dependency plays an important role in the Fifth normal form, also known as project-join normal form, because it can be proven that if a scheme R {displaystyle R} is decomposed in tables R 1 {displaystyle R_{1}} to R n {displaystyle R_{n}} , the decomposition will be a lossless-join decomposition if the legal relations on R {displaystyle R} are restricted to a join dependency on R {displaystyle R} called ∗ ( R 1 , R 2 , … , R n ) {displaystyle *(R_{1},R_{2},ldots ,R_{n})} .