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The RDBMS must take care that modifications to the parent table does not break the referential integrity which could happen if an item in the parent table is deleted that is referred to by a foreign key.
As an example; In a database that consists of employees and departments, it is then useful to know in which department each employee work. A foreign key "Department#" could be created in the Employee table that was only allowed to refer to a primary key in the Department table.
See also: entity integrity