A database trigger is procedural code that is automatically executed in response to certain events on a particular table or view in a database. The trigger is mostly used for maintaining the integrity of the information on the database. For example, when a new record (representing a new worker) is added to the employees table, new records should also be created in the tables of the taxes, vacations and salaries.
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Some articles on database trigger:
... An example of implementation of triggers in non-relational database can be Sedna, that provides support for triggers based on XQuery ... Triggers in Sedna were designed to be analogous to SQL2003 triggers, but natively base on XML query and update languages (XPath, XQuery and XML update language) ... A trigger in Sedna is set on any nodes of an XML document stored in database ...