Editing Entity integrity

{{Short description|Concept in relational databases}}
'''Entity integrity''' is concerned with ensuring that each row of a table has a unique and non-null primary key value; this is the same as saying that each row in a table represents a single instance of the entity type modelled by the table. A requirement of E. F. Codd in his seminal paper is that a primary key of an entity, or any part of it, can never take a null value.<ref>{{Cite web|url=https://www.sqa.org.uk/e-learning/MDBS01CD/page_39.htm|title=Entity Integrity}}</ref>
The relational model states that every [[relation (database)|relation]] (or [[table (database)|table]]) must have an identifier, called the [[primary key]] (abbreviated PK), in such a way that every [[row (database)|row]] of the same relation be identifiable by its content, that is, by a unique and minimal value. The PK is a not empty set of [[attribute (computing)|attributes]] (or [[column]]s). The same format applies to the [[foreign key]] (abbreviated FK) because each FK matches a preexistent PK. Each of attributes being part of a PK (or of a FK) must have data values (such as numbers, letters or typographic symbols) but not [[data marks]] (also known as [[NULL marks]] in [[SQL]] world). 
Morphologically, a [[Compound key|composite primary key]] is in a "steady state": If it is reduced, PK will lose its property of identifying every row of its relation but if it is extended, PK will be redundant.

==See also==
*[[Domain integrity]]
*[[Data integrity]]
*[[Referential integrity]]
*[[Null (SQL)]]

==References==
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[[Category:Data modeling]]
[[Category:Data quality]]


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