In 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 to 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.
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| - Propagation constraint (en)
- 传播约束 (zh)
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| - 在数据库系统中，传播约束（propagation constraint）“详细说明了当我们更新目标表的一行或多行时相关表应该发生的情况”。表使用主键与外键关系实现链接。用户可能会更新其中的一个表导致这种关系不再保持一致，这称为破坏了参照完整性。例如，如果员工表包含“家庭用品部门”的部门编号，它是部门表的外键，并且用户从部门表中删除了该部门，则家庭用品部门员工记录将引用不再存在的部门编号。 关系数据库管理系统解决此问题的方法是传播约束，以确保表之间的关系保持无错。Beynon-Davies列出了关系数据库管理系统处理删除目标和关联元组的三种方式：
* Restricted Delete - 在所有（通过外键）指向它的行都被删除之前，用户不能删除目标行。这意味着在从部门表中删除部门之前，需要删除所有家庭用品部门员工，或者更改他们的部门。
* Cascades Delete - 可以删除目标行且所有（通过外键）指向它的行也被删除。
* Nullifies Delete - 可以删除目标行并把所有（指向它的）外键都置为空。在这种情况下，删除家庭用品部门后，在该部门工作的员工的部门将具有NULL（未知）值。 (zh)
- In 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 to 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. (en)
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| - In 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 to 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-Davies explains the three ways that RDBMS handle deletions of target and related tuples:
* Restricted Delete - the user cannot delete the target row until all rows that point to it (via foreign keys) have been deleted. This means that all Housewares employees would need to be deleted, or their departments 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. In this case, after removing the housewares department, employees who worked in this department would have a NULL (unknown) value for their department. (en)
- 在数据库系统中，传播约束（propagation constraint）“详细说明了当我们更新目标表的一行或多行时相关表应该发生的情况”。表使用主键与外键关系实现链接。用户可能会更新其中的一个表导致这种关系不再保持一致，这称为破坏了参照完整性。例如，如果员工表包含“家庭用品部门”的部门编号，它是部门表的外键，并且用户从部门表中删除了该部门，则家庭用品部门员工记录将引用不再存在的部门编号。 关系数据库管理系统解决此问题的方法是传播约束，以确保表之间的关系保持无错。Beynon-Davies列出了关系数据库管理系统处理删除目标和关联元组的三种方式：
* Restricted Delete - 在所有（通过外键）指向它的行都被删除之前，用户不能删除目标行。这意味着在从部门表中删除部门之前，需要删除所有家庭用品部门员工，或者更改他们的部门。
* Cascades Delete - 可以删除目标行且所有（通过外键）指向它的行也被删除。
* Nullifies Delete - 可以删除目标行并把所有（指向它的）外键都置为空。在这种情况下，删除家庭用品部门后，在该部门工作的员工的部门将具有NULL（未知）值。 (zh)
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