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Concept
Intégrité référentielle
Résumé
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 (either in the same or a different relation), then the referenced value must exist.
For referential integrity to hold in a relational database, any column in a base table that is declared a foreign key can only contain either null values or values from a parent table's primary key or a candidate key. In other words, when a foreign key value is used it must reference a valid, existing primary key in the parent table. For instance, deleting a record that contains a value referred to by a foreign key in another table would break referential integrity. Some relational database management systems (RDBMS) can enforce referential integrity, normally either by deleting the foreign key rows as well to maintain integrity, or by returning an error and not performing the delete. Which method is used may be determined by a referential integrity constraint defined in a data dictionary.
The adjective 'referential' describes the action that a foreign key performs, 'referring' to a linked column in another table. In simple terms, 'referential integrity' guarantees that the target 'referred' to will be found. A lack of referential integrity in a database can lead relational databases to return incomplete data, usually with no indication of an error.
An inclusion dependency over two (possibly identical) predicates and from a schema is written , where the , are distinct attributes (column names) of and . It implies that the tuples of values appearing in columns for facts of must also appear as a tuple of values in columns for some fact of .
Such constraint is a particular form of tuple-generating dependency (TGD) where in both the sides of the rule there is only one relational atom. In first-order logic it is expressible as , where is the vector (whose size is ) of variables shared by and , and no variable appears multiple times neither in the TGD's body nor in its head.
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