Editing Entity–relationship model (section)

===Cardinalities===
Modifications to the original specification can be beneficial. Chen described [[look-across cardinality|look-across cardinalities]]. As an aside, the [[Barker's notation|Barker–Ellis]] notation, used in Oracle Designer, uses same-side for minimum cardinality (analogous to optionality) and role, but look-across for maximum cardinality (the crow's foot).{{clarify|This whole section is almost incomprehensible!  Please define 'cardinalities', 'look-across', etc.|date=April 2012}}

Research by [[Merise]], Elmasri & Navathe and others has shown there is a preference for same-side for roles and both minimum and maximum cardinalities,<ref>Hubert Tardieu, Arnold Rochfeld and René Colletti La methode MERISE: Principes et outils (Paperback - 1983)</ref><ref>Elmasri, Ramez, B. Shamkant, Navathe, Fundamentals of Database Systems, third ed., Addison-Wesley, Menlo Park, CA, USA, 2000.</ref><ref>{{Cite book |last1=Atzeni |first1=Paolo |url=https://books.google.com/books?id=odZK99osY1EC&q=genova&pg=PA52 |title=ER 2004 : 23rd International Conference on Conceptual Modeling, Shanghai, China, November 8-12, 2004 |last2=Chu |first2=Wesley |last3=Lu |first3=Hongjun |last4=Ling |first4=Tok Wang |last5=Zhou |first5=Shuigeng |date=2004-10-27 |isbn=9783540237235}}</ref> and researchers (Feinerer, Dullea et al.) have shown that this is more coherent when applied to n-ary relationships of order greater than&nbsp;2.<ref>{{Cite web |title=A Formal Treatment of UML Class Diagrams as an Efficient Method for Configuration Management 2007 |url=http://publik.tuwien.ac.at/files/pub-inf_4582.pdf}}</ref><ref>{{Cite web |title=James Dullea, Il-Yeol Song, Ioanna Lamprou - An analysis of structural validity in entity-relationship modeling 2002 |url=http://www.ischool.drexel.edu/faculty/song/publications/p_DKE_03_Validity.pdf |archive-url=https://web.archive.org/web/20090424030434/http://www.ischool.drexel.edu/faculty/song/publications/p_DKE_03_Validity.pdf |url-status=dead |archive-date=April 24, 2009 }}</ref>

Dullea et al. states: "A 'look across' notation such as used in the [[Unified Modelling Language|UML]] does not effectively represent the semantics of participation constraints imposed on relationships where the degree is higher than binary."

Feinerer says: "Problems arise if we operate under the look-across semantics as used for UML associations. Hartmann<ref>Hartmann, Sven. "[http://crpit.com/confpapers/CRPITV17Hartmann.pdf Reasoning about participation constraints and Chen's constraints] {{Webarchive|url=https://web.archive.org/web/20130510031920/http://crpit.com/confpapers/CRPITV17Hartmann.pdf |date=2013-05-10 }}". Proceedings of the 14th Australasian database conference-Volume 17. Australian Computer Society, Inc., 2003.</ref> investigates this situation and shows how and why different transformations fail." ''(Although the "reduction" mentioned is spurious as the two diagrams 3.4 and 3.5 are in fact the same)'' and also "As we will see on the next few pages, the look-across interpretation introduces several difficulties that prevent the extension of simple mechanisms from binary to n-ary associations."

[[File:ERD Representation.svg|thumb|320px|Various methods of representing the same one to many relationship. In each case, the diagram shows the relationship between a person and a place of birth: each person must have been born at one, and only one, location, but each location may have had zero or more people born at it.]]
[[Image:ERD-artist-performs-song.svg|thumb|320px|Two related entities shown using Crow's Foot notation. In this example, an optional relationship is shown between Artist and Song; the symbol composed of branching lines, closest to the song entity represents "zero, one, or many", whereas a song has "one and only one" Artist, emphasized by the symbol composed of parallel lines. The former is therefore read as, an Artist (can) perform(s) "zero, one, or many" song(s).]] Chen's notation for entity–relationship modeling uses rectangles to represent entity sets, and diamonds to represent relationships appropriate for [[first-class object]]s: they can have attributes and relationships of their own. If an entity set participates in a relationship set, they are connected with a line.

Attributes are drawn as ovals and connected with a line to exactly one entity or relationship set.

Cardinality constraints are expressed as follows:
* a double line indicates a ''participation constraint'', [[total relation|totality]], or [[surjective function|surjectivity]]: all entities in the entity set must participate in ''at least one'' relationship in the relationship set;
* an arrow from an entity set to a relationship set indicates a [[unique key|key constraint]], i.e. [[injective relation|injectivity]]: each entity of the entity set can participate in ''at most one'' relationship in the relationship set;
* a thick line indicates both, i.e. [[bijection|bijectivity]]: each entity in the entity set is involved in ''exactly one'' relationship.
* an underlined name of an attribute indicates that it is a [[unique key|key]]: two different entities or relationships with this attribute always have different values for this attribute.

Attributes are often omitted as they can clutter up a diagram. Other diagram techniques often list entity attributes within the rectangles drawn for entity sets.