Editing Foreign key (section)

{{Short description|Concept in database systems}}
A '''foreign key''' is a set of [[Attribute (database)|attributes]] in a [[table (database)|table]] that refers to the [[primary key]] of another table, linking these two tables. In the context of [[relational database]]s, a foreign key is subject to an [[inclusion dependency]] constraint that the [[tuple]]s consisting of the foreign key attributes in one [[Relation (database)|relation]], R, must also exist in some other (not necessarily distinct) relation, S; furthermore that those attributes must also be a [[candidate key]] in S.<ref>{{cite book|last=Coronel|first=Carlos|title=Database Systems: Design, Implementation, and Management|year=2010|publisher=South-Western/Cengage Learning|location=Independence KY|isbn=978-0-538-74884-1|page=65}}</ref><ref name=elmasri>{{cite book|last=Elmasri|first=Ramez|title=Fundamentals of Database Systems|url=https://archive.org/details/fundamentalsdata00elma|url-access=limited|year=2011|publisher=Addison-Wesley|isbn=978-0-13-608620-8|pages=[https://archive.org/details/fundamentalsdata00elma/page/n101 73]–74}}</ref><ref>{{cite book|last=Date|first=C. J.|title=A guide to the SQL standard|year=1996|publisher=Addison-Wesley|isbn=978-0201964264|page=206}}</ref>

In other words, a foreign key is a set of attributes that {{em|references}} a candidate key. For example, a table called TEAM may have an attribute, MEMBER_NAME, which is a foreign key referencing a candidate key, PERSON_NAME, in the PERSON table. Since MEMBER_NAME is a foreign key, any value existing as the name of a member in TEAM must also exist as a person's name in the PERSON table; in other words, every member of a TEAM is also a PERSON.