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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 consisting of the foreign key attributes in one relation, R, must also exist in some other (not necessarily distinct) relation, S; furthermore that those ...
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. [2] In other words, when a foreign key value is used it must reference a valid, existing primary key in the parent table. For ...
Here is an example of a primary key becoming a foreign key on a related table. ID migrates from the Author table to the Book table. Author Table Schema : Author ( ID , Name , Address , Born ) Book Table Schema : Book ( ISBN , AuthorID , Title , Publisher , Price )
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. [1] The relational model states that every relation (or table ) must have an identifier, called the primary key (abbreviated PK), in such a way that every row of the same relation be identifiable by its content ...
For example, the "Doctor Last Name" field might be assigned as a primary key of the Doctor table with all people having same last name organized alphabetically according to the first three letters of their first name. A table can also have a foreign key which indicates that field is linked to the primary key of another table. [2]
An example of breaking referential integrity: if a table of employees includes a department number for 'Housewares' which is a foreign key to a table of departments and a user deletes that department from the department table then Housewares employees records would refer to a non-existent department number.
An example of a data-integrity mechanism is the parent-and-child relationship of related records. If a parent record owns one or more related child records all of the referential integrity processes are handled by the database itself, which automatically ensures the accuracy and integrity of the data so that no child record can exist without a parent (also called being orphaned) and that no ...
A surrogate key (or synthetic key, pseudokey, entity identifier, factless key, or technical key [citation needed]) in a database is a unique identifier for either an entity in the modeled world or an object in the database. The surrogate key is not derived from application data, unlike a natural (or business) key. [1]