Editing First normal form (section)

== Controversy about compound values ==
There is some discussion about to what extent compound or complex values other than relations (such as [[Array (data structure)|arrays]] or [[XML]] data) are permitted in 1NF.{{Citation needed|date=May 2025}} Codd states that relations are the only type of compound data allowed within the relational model (if not in attribute domains), since any additional type of compound data would add complexity without adding power; nevertheless, the model specifically allows "certain special functions" like <code>SUBSTRING</code> to decompose values otherwise considered atomic.{{r|Codd 1990}}{{rp|page=6,340}}

[[Hugh Darwen]] and [[Christopher J. Date]] have suggested that Codd's concept of an "atomic value" is ambiguous, and that this ambiguity has led to widespread confusion about how 1NF should be understood.<ref>Darwen, Hugh. "Relation-Valued Attributes; or, Will the Real First Normal Form Please Stand Up?", in C. J. Date and Hugh Darwen, ''Relational Database Writings 1989-1991'' (Addison-Wesley, 1992).</ref><ref>{{cite book |last=Date |first=C. J. |author-link=Christopher J. Date |chapter=Chapter 8: What First Normal Form Really Means |date=2007 |title=Date on Database: Writings 2000–2006 |publisher=Apress |isbn=978-1-4842-2029-0 |page=108 |quote='[F]or many years,' writes Date, 'I was as confused as anyone else. What's worse, I did my best (worst?) to spread that confusion through my writings, seminars, and other presentations.'}}</ref> In particular, the notion of an atomic value as a "value that cannot be decomposed" is problematic, as it would seem to imply that few, if any, data types are atomic:
*A [[String (computer science)|string]] would seem not to be atomic, as an RDBMS typically provides operators to decompose it into [[substring]]s.
*A [[Fixed-point arithmetic|fixed-point]] number would seem not to be atomic, as an RDBMS typically provides operators to decompose it into integer and fractional components.
* An [[ISBN]] would seem not to be atomic, as it includes various parts, including the ''registration group'', ''registrant'' and ''publication'' elements.
Date suggests that "the notion of atomicity ''has no absolute meaning''":<ref name="Date 2007">{{cite book |last=Date| first=C. J. |author-link=Christopher J. Date |chapter=Chapter 8: What First Normal Form Really Means |date=2007 |title=Date on Database: Writings 2000–2006 |publisher=Apress |isbn=978-1-4842-2029-0}}</ref>{{rp|page=112}}<ref>{{cite book |last=Date |first=C. J. |author-link=Christopher J. Date |url=https://books.google.com/books?id=BCjkCgAAQBAJ&pg=PA50 |title=SQL and Relational Theory: How to Write Accurate SQL Code |date=6 November 2015 |publisher=O'Reilly Media |isbn=978-1-4919-4115-7 |pages=50– |access-date=31 October 2018}}</ref>{{Pages needed|date=May 2025}} a value may be considered atomic for some purposes, but may be considered an assemblage of more basic elements for other purposes. If this position is accepted, 1NF cannot be defined with reference to atomicity. Columns containing any conceivable data type (from strings and numeric types to arrays and tables) are then acceptable in a 1NF table,{{Citation needed|date=May 2025}} although perhaps not always desirable – for example, it may be desirable to separate a CustomerName column into two columns, FirstName and Surname.