Sunday, June 26, 2022

Repeating Groups and 1NF (T&N)



“A commonly used example of a table that is not in 2-NF is one with repeated attributes (i.e. child1, child2, child3). However, after examining the definition of 2NF in your book PRACTICAL ISSUES IN DATABASE MANAGEMENT, it seems to me that tables such as these do in fact satisfy 2NF. Am I missing something?” --Reader

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Then (11/15/02): MORE ON REPEATING GROUPS AND NORMALIZATION

Very important: The determination of the normalization level of any given table can be made only with respect to a set of business rules that specify dependencies among the attributes represented by the columns!!! It is impossible to determine that from just the table without knowing the rules. Thus, to determine whether your table is in 2NF I would need the whole table and the business rules, including dependencies.

Except that you're actually referring to 1NF, not 2NF. R-tables are in 1NF by definition because they do not have the table equivalent of repeating groups -- multivalued columns. Had your table had a CHILDREN multivalued column, it would not be in 1NF and, thus, it would not be a R-table.

The table in your example is poorly designed, but it is in 1NF. This is a common error induced even by industry "experts". Here’s, for example, Joe Celko in a very old issue of DBMS magazine, which I had debunked in an article at that time.

"The relational model states that in order to be in 1NF, a table can have no repeating groups. But since SQL does not support repeating groups, the only way people can write a repeating group in SQL ... is:

CREATE TABLE employees
 (emp# INTEGER PRIMARY KEY,
  ename CHAR(15) NOT NULL,
  child1 CHAR(15),
  child2 CHAR(15),
    :       :
  childN CHAR(15));”

& Now

You have been contaminated with a common misconception in the industry: the confusion of "repeating group" with "repeated attribute". Moreover, the question is really about 1NF, not 2NF (beyond 1NF the normal form of a relation can only be determined with reference to the attribute dependencies defined by the conceptual model that the database is intended to represent).

According to the current understanding of the RDM, database relations are by definition in both 1NF and 5NF (tables only visualize database relations, they play no role in the RDM). Adherence to three core design principles produces 1NF+5NF databases, obviating normalization (to 1NF) and further normalization (to 5NF), which come up only as repair of poor designs (2NF-4NF).

A relation is not in 1NF (and, thus, not a database relation) if it has relation-valued attributes (RVA) defined on relation-valued domains (RVD), or "nested relations" -- the relational equivalent to hierarchic "repeating groups" (i.e., groups of values that repeat).

Repeated attributes (i.e., an attribute of the same type that repeats, here CHILD) are poor design, but do not violate 1NF. A CHILDREN RVA with relation values would: each relation value a set of tuples (groups of values) representing the properties of children that repeat for each employee; in which case EMPLOYEES would not be a database relation.

Further Reading

First Normal Form in Theory and Practice series

Simple Domains and Value Atomicity

 

 

 

 

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