Бесінші нормативтік форма: Деректер базасындағы қайталануды жою

Fifth normal form (5NF), also known as projection–join normal form (PJ/NF), is a level of database normalization designed to remove redundancy in relational databases recording multi valued facts by isolating semantically related multiple relationships. A table is said to be in the 5NF if and only if every non trivial join dependency in that table is implied by the candidate keys. It is the final normal form as far as removing redundancy is concerned. A 6NF also exists, but its purpose is not to remove redundancy and it is therefore only adopted by a few data warehouses, where it can be useful to make tables irreducible. A join dependency *{A, B, Z} on R is implied by the candidate key(s) of R if and only if each of A, B, , Z is a superkey for R.

The fifth normal form was first described by Ronald Fagin in his 1979 conference paper Normal forms and relational database operators.

Only in rare situations does a 4NF table not conform to 5NF; for instance, when the decomposed tables are cyclic. These are situations in which a complex real world constraint governing the valid combinations of attribute values in the 4NF table is not implicit in the structure of that table. If such a table is not normalized to 5NF, the burden of maintaining the logical consistency of the data within the table must be carried partly by the application responsible for insertions, deletions, and updates to it; and there is a heightened risk that the data within the table will become inconsistent. In contrast, the 5NF design excludes the possibility of such inconsistencies. A table T is in fifth normal form (5NF) or projection join normal form (PJ/NF) if it cannot have a lossless decomposition into any number of smaller tables. The case where all the smaller tables after the decomposition have the same candidate key as the table T is excluded.