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Third Normal Form (3NF)

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Third Normal Form (3NF) - definition(s)

Third Normal Form (3NF) : (1) A relation R is in third normal form (3NF) if and only if it is in 2NF and every nonkey attribute is nontransitively dependent upon the primary key. (2) A table is in 3NF if each nonkey column provides a fact that is dependent only on the entire key of the table.

[Category=Data Quality ]

Source: Larry English, External , 12-Feb-2009 10:51

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Third normal form (3NF) - In relational data analysis, a relation is in third normal form if it is in second normal form and no transitive dependencies exist.

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Source: DAMA UK, 20 July 2011 10:23:16, External

third normal form - [database structures] The third level of guidelines for designing table and data structures in a relational database. The third-normal-form guideline incorporates the guidelines of first and second normal form; in addition, it recommends removing from a table those columns that do not depend on the table's primary key. A database that follows these guidelines is said to be in third normal form.

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Source: esri, 11 September 2012 10:05:52, External 

3NF - In creating a database, normalization is the process of organizing it into tables in such a way that the results of using the database are always unambiguous and as intended. Normalization may have the effect of duplicating data within the database and often results in the creation of additional tables. (While normalization tends to increase the duplication of data, it does not introduce redundancy, which is unnecessary duplication.) Normalization is typically a refinement process after the initial exercise of identifying the data objects that should be in the database, identifying their relationships, and defining the tables required and the columns within each table.

A simple example of normalizing data might consist of a table showing:

   Customer Item purchased Purchase price
   Thomas    Shirt          $40
   Maria     Tennis shoes   $35
   Evelyn    Shirt          $40
   Pajaro    Trousers       $25

If this table is used for the purpose of keeping track of the price of items and you want to delete one of the customers, you will also delete a price. Normalizing the data would mean understanding this and solving the problem by dividing this table into two tables, one with information about each customer and a product they bought and the second about each product and its price. Making additions or deletions to either table would not affect the other.

Normalization degrees of relational database tables have been defined and include:

First normal form (1NF). This is the "basic" level of normalization and generally corresponds to the definition of any database, namely:

   * It contains two-dimensional tables with rows and columns.
   * Each column corresponds to a sub-object or an attribute of the object represented by the entire table.
   * Each row represents a unique instance of that sub-object or attribute and must be different in some way from any other row (that is, no duplicate rows are possible).
   * All entries in any column must be of the same kind. For example, in the column labeled "Customer," only customer names or numbers are permitted.

Second normal form (2NF). At this level of normalization, each column in a table that is not a determiner of the contents of another column must itself be a function of the other columns in the table. For example, in a table with three columns containing customer ID, product sold, and price of the product when sold, the price would be a function of the customer ID (entitled to a discount) and the specific product.

Third normal form (3NF). At the second normal form, modifications are still possible because a change to one row in a table may affect data that refers to this information from another table. For example, using the customer table just cited, removing a row describing a customer purchase (because of a return perhaps) will also remove the fact that the product has a certain price. In the third normal form, these tables would be divided into two tables so that product pricing would be tracked separately.

Domain/key normal form (DKNF). A key uniquely identifies each row in a table. A domain is the set of permissible values for an attribute. By enforcing key and domain restrictions, the database is assured of being freed from modification anomalies. DKNF is the normalization level that most designers aim to achieve.

Related glossary terms: BLOB (binary large object), data structure, catalog, data mart (datamart), ECMAScript (European Computer Manufacturers Association Script), Visual FoxPro, segment, block, flat file

[Category=Data Management ]

Source:, 02 July 2013 09:01:59, External





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