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Explain multivalued dependency and fourth normal form,with an example .Database Management Systems - 10CS54 VTU notes (JULY 2013 / July 2015)

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Multivalued Dependencies and Fourth Normal Form (4NF) 4NF associated with a dependency called multi-valued dependency (MVD). MVDs in a relation are due to first normal form (1NF), which disallows an attribute in a row from having a set of values. MVD represents a dependency between attributes (for example, A, B, and C) in a relation, such that for each value of A there is a set of values for B, and a set of values for C. However, the set of values for B and C are independent of each other.MVD between attributes A, B, and C in a relation using the following notation A - > - > B (A multidetermines B) A - > - > C

Formal Definition of Multivalued Dependency A multivalued dependency (MVD) X Y specified on R, where X, and Y are both subsets of R and Z = (R – (X Y)) specifies the following restrictions on r(R) t3[X]=t4[X]=t1[X]=t2[X] t3[Y] = t1[Y] and t4[Y] = t2[Y] t3[Z] = t2[Z] and t4[Z] = t1 [Z] 

Fourth Normal Form (4NF) A relation that is in Boyce-Codd Normal Form and contains no MVDs. BCNF to 4NF involves the removal of the MVD from the relation by placing the attribute(s) in a new relation along with a copy of the determinant(s). 


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