<P> Referential integrity is a property of data stating references within it are valid . In the context of relational databases, it requires every value of one attribute (column) of a relation (table) to exist as a value of another attribute (column) in a different (or the same) relation (table). </P> <P> For referential integrity to hold in a relational database, any column in a base table that is declared a foreign key can contain either a null value, or only 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 . </P>

Briefly explain what is meant by referential integrity constraint