June 2015 Archive


Modification of Constraints

It is possible to add, modify, or delete constraints at any time. The way to express such modifications depends on whether the constraint involved is associated with an attribute, a table, or a database schema.

Schema-Level Constraints and Triggers

The most powerful forms of active elements in SQL are not associated with specific tuples or components of tuples. These elements, called "triggers" and "assertions," are part of the database schema, on a par with the relations and views themselves.

Triggers in SQL

The SQL trigger statement gives the user a number of different options in the event, condition, and action parts. Here are the principal features. 1. The action may be executed either before or after the triggering event. 2. The action can refer to both old and/or new values of tuples that were inserted, deleted, or updated in the event that triggered the action.

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