View source for One-to-many (data model)

{{Short description|Relationship between entities}}

In [[systems analysis]], a '''one-to-many''' relationship is a type of [[cardinality (data modeling)|cardinality]] that refers to the relationship between two [[wikt:entity|entities]] (see also [[entity–relationship model]]). For example, take a car and an owner of the car. The car can only be owned by one owner at a time or not owned at all, and an owner could own zero, one, or multiple cars. One owner could have many cars, one-to-many.

In a [[relational database]], a one-to-many relationship exists when one record is related to many records of another table. A one-to-many relationship is not a property of the data, but rather of the relationship itself. One-to-many often refer to a primary key to foreign key relationship between two tables, where the record in the first table can relate to multiple records in the second table. A foreign key is one side of the relationship that shows a row or multiple rows, with one of those rows being the primary key already listed on the first table. This is also called a [[Foreign key|foreign key constraint]], which is important to keep data from being duplicated and have relationships within the database stay reliable as more information is added.

Many-to-many relationships are not able to be used in relational databases and must be converted to one-to-many relationships. Both one-to-many and one-to-one relationships are common in relational databases but are normally created majorly with one-to-many relationships. <ref>{{Cite web |date=1 March 2024 |title=Three Relationship Types |url=https://www.relationaldbdesign.com/database-design/module6/three-relationship-types.php |access-date=1 March 2024 |website=RelationalDBDesign}}</ref>

The opposite of one-to-many is '''many-to-one'''. The [[converse relation|transpose]] of a one-to-many relationship is a many-to-one relationship.

== Entity relationship diagram (ERD) notations ==
One notation as described in [[entity–relationship model|Entity Relationship modeling]] is '''Chen notation''' or formally '''Chen ERD notation''' created originally by [[Peter Chen]] in 1976 where a '''one-to-many''' relationship is notated as '''1:N''' where '''N''' represents the cardinality and can be 0 or higher.
A '''many-to-one''' relationship is sometimes notated as '''N:1'''.<ref>{{cite web |last1=Botting |first1=Richard |title=Entity Relationship Models |url=http://www.csci.csusb.edu/dick/cs557/d3.html |access-date=27 July 2021}}</ref> 

== See also ==
* [[One-to-one (data model)]]
* [[Many-to-many (data model)]]

==References==
<references/>

[[Category:Data modeling]]


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