AIn database systems, a '''propagation constraint''' "details what should happen to a related table when we update a row or rows of a target table" (Paul Beynon-Davies, 2004, p.108). Tables are linked using [[primary key]] to [[foreign key]] relationships. It is possible for users update one table in a relationship in such a way that the relationship is no longer consistent and this is known as breaking [[referential integrity]]. 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.
Propagation constraints are methods used by 'Relational Database Management Systems' ([[RDBMS]]) to solve this problem by ensuring that relationships between tables are preserved without error. In his database textbook, Beynon-Daives explains the three ways that RDBMS handle deletions of target and related [[tuple]]s:
