Propagation constraint: Difference between revisions

Revision as of 20:17, 5 June 2007

Revision as of 17:02, 7 May 2008

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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:

* Restricted Delete - the user cannot delete the target row until all rows that point to it (via [[foreign key]]s) have been deleted. This means that all Housewares employees would need to be deleted, byor thetheir userdepartments changed, before removing the department from the departmental table.

* Cascades Delete - can delete the target row and all rows that point to it (via foreign keys) are also deleted. The process is the same as a restricted delete, except that the RDBMS would delete the Houseware employees automatically before removing the department.

* Nullifies Delete - can delete the target row and all foreign keys (pointing to it) are set to [[Null (SQL)|null]]. In this case, after removing the housewares department, employees who worked in this department would have a [[Null (SQL)|NULL]] (unknown) value for their department.