Referential integrity
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Referential integrity in a relational database is consistency between coupled tables. Referential integrity is usually enforced by the combination of a primary key and a foreign key. For referential integrity to hold, any field in a table that is declared a foreign key can contain only values from a parent table's primary key field. For instance, deleting a record that contains a value referred to by a foreign key in another table would break referential integrity. The relational database management system (RDBMS) enforces referential integrity.
Example
An employee database stores the department in which each employee works. The field "DepartmentNumber" in the Employee table is declared a foreign key, and it refers to the field "Index" in the Department table which is declared a primary key. Referential integrity would be broken by deleting a department from the Department table if employees listed in the Employee table are listed as working for that department.