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.
The relational model states that every
relation
Relation or relations may refer to:
General uses
* International relations, the study of interconnection of politics, economics, and law on a global level
* Interpersonal relationship, association or acquaintance between two or more people
* ...
(or
table
Table may refer to:
* Table (database), how the table data arrangement is used within the databases
* Table (furniture), a piece of furniture with a flat surface and one or more legs
* Table (information), a data arrangement with rows and column ...
) must have an identifier, called the
primary key
In the relational model of databases, a primary key is a designated attribute (column) that can reliably identify and distinguish between each individual record in a table. The database creator can choose an existing unique attribute or combinati ...
(abbreviated PK), in such a way that every
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
attributes
Attribute may refer to:
* Attribute (philosophy), a characteristic of an object
* Attribute (research), a quality of an object
* Grammatical modifier
In linguistics, a modifier is an optional element in phrase structure or clause structure whic ...
(or
column
A column or pillar in architecture and structural engineering is a structural element that transmits, through compression, the weight of the structure above to other structural elements below. In other words, a column is a compression member ...
s). The same format applies to the
foreign key
A foreign key is a set of attributes in a table that refers to the primary key of another table, linking these two tables. In the context of relational databases, a foreign key is subject to an inclusion dependency constraint that the tuples ...
(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
Structured Query Language (SQL) (pronounced ''S-Q-L''; or alternatively as "sequel")
is a domain-specific language used to manage data, especially in a relational database management system (RDBMS). It is particularly useful in handling s ...
world).
Morphologically, a
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
Data integrity is the maintenance of, and the assurance of, data accuracy and consistency over its entire Information Lifecycle Management, life-cycle. It is a critical aspect to the design, implementation, and usage of any system that stores, proc ...
*
Referential integrity
Referential integrity is a property of data stating that all its references are valid. In the context of relational databases, it requires that if a value of one attribute (column) of a relation (table) references a value of another attribute (e ...
*
Null (SQL)
In SQL, null or NULL is a special marker used to indicate that a data value does not exist in the database. Introduced by the creator of the Relational model, relational database model, E. F. Codd, SQL null serves to fulfill the requirement ...
References
Data modeling
Data quality
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