4nf การใช้

• The multivalued dependency plays a role in the 4NF database normalization.
• Ronald Fagin demonstrated that it is always possible to achieve 4NF . Rissanen's theorem is also applicable on multivalued dependencies.
• To eliminate the possibility of these anomalies, we must place the facts about varieties offered into a different table from the facts about delivery areas, yielding two tables that are both in 4NF:
• Also note that the table is in 4NF, since there are no multivalued dependencies ( 2-part join dependencies ) in the table : no column ( which by itself is not a candidate key or a superkey ) is a determinant for the other two columns.
• Therefore it meets all normal forms up to BCNF . If we assume, however, that pizza varieties offered by a restaurant are not affected by delivery area ( i . e . a restaurant offers all pizza varieties it makes to all areas it supplies ), then it does not meet 4NF . The problem is that the table features two non-trivial multivalued dependencies on the { Restaurant } attribute ( which is not a superkey ).