(The trigger is called a BEFORE statement trigger or statement-level BEFORE trigger.)
CREATE TRIGGER AFTER 100 UPDATES UPDATE
To create a trigger that fires in response to a MERGE statement, create triggers on the INSERT and UPDATE statements to which the MERGE operation decomposes.Ī DML trigger is either simple or compound.Ī simple DML trigger fires at exactly one of these timing points: To enforce complex business or referential integrity rules that you cannot define with constraintsĪ DML trigger is created on either a table or view, and its triggering event is composed of the DML statements DELETE, INSERT, and UPDATE. To enforce referential integrity when child and parent tables are on different nodes of a distributed database Oracle strongly recommends that you use triggers to constrain data input only in these situations:
However, triggers can enforce some complex business rules that constraints cannot. For example, a trigger can prevent a DML statement from inserting a NULL value into a database column, but the column might contain NULL values that were inserted into the column before the trigger was defined or while the trigger was disabled.Ĭonstraints are easier to write and less error-prone than triggers that enforce the same rules. For example, if a database column has a NOT NULL constraint, then its existing data is NOT NULL and no DML statement can violate the NOT NULL constraint.Ī trigger applies only to new data. For more information, see Oracle Database Vault Administrator's Guide.īoth triggers and constraints can constrain data input, but they differ significantly.Ī constraint applies to both existing and new data. For high-assurance security, use Oracle Database Vault. Triggers are not reliable security mechanisms, because they are programmatic and easy to disable. Updates in the trigger wait for existing data locks to be released before proceeding. Queries in the trigger see the current read-consistent materialized view of referenced tables and any data changed in the same transaction. SQL statements running in triggers follow the same rules that standalone SQL statements do.
When a trigger fires, tables that the trigger references might be undergoing changes made by SQL statements in other users' transactions. For more information about the WHEN clause, see "WHEN ( condition)". For more information, see "System Triggers".Ī conditional trigger has a WHEN clause that specifies a SQL condition that the database evaluates for each row that the triggering statement affects. If the trigger is created on a schema or the database, then the triggering event is composed of either DDL or database operation statements, and the trigger is called a system trigger. For more information, see "DML Triggers". If the trigger is created on a table or view, then the triggering event is composed of DML statements, and the trigger is called a DML trigger. For more information about the CREATE TRIGGER statement, see "CREATE TRIGGER Statement". By default, a trigger is created in the enabled state. You also specify the timing point, which determines whether the trigger fires before or after the triggering statement runs and whether it fires for each row that the triggering statement affects. The trigger is said to be created on or defined on the item, which is either a table, a view, a schema, or the database.
You specify the triggering event in terms of triggering statements and the item on which they act. You create a trigger with the CREATE TRIGGER statement. While a trigger is disabled, it does not fire. While a trigger is enabled, the database automatically invokes it-that is, the trigger fires-whenever its triggering event occurs. Unlike a stored procedure, you can enable and disable a trigger, but you cannot explicitly invoke it. Like a stored procedure, a trigger is a named PL/SQL unit that is stored in the database and can be invoked repeatedly.