One aspect of triggers that can be difficult to understand is the order in which triggers fire if several triggers are impacted by the same triggering action. Whether competing triggers are fired, and the order in which they are fired, depends on two things: trigger type (BEFORE, INSTEAD OF, or AFTER), and trigger scope (row-level or statement-level).
UPDATE statements can modify column values in more than one table. The sequence of trigger firing is the same for each table, but the order that the tables are updated is not guaranteed.
For row-level triggers, BEFORE triggers fire before INSTEAD OF triggers, which fire before AFTER triggers. All row-level triggers for a given row fire before any triggers fire for a subsequent row.
For statement-level triggers, INSTEAD OF triggers fire before AFTER triggers. Statement-level BEFORE triggers are not supported.
If there are competing statement-level and row-level AFTER triggers, the statement-level AFTER triggers fire after all row-level triggers have completed.
If there are competing statement-level and row-level INSTEAD OF triggers, the row-level triggers do not fire.
The OLD and NEW temporary tables created for AFTER STATEMENT triggers have the same schema as the underlying base table, with the same column names and data types. However these tables do not have primary keys, foreign keys, or indexes. The order of the rows in the OLD and NEW temporary tables is not guaranteed and may not match the order in which the base table rows were updated originally.
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