You may need to set up the following data structures for this example to work: The actions then insert new rows or update existing rows, as appropriate. If the object is read only, then it is not necessary to define triggers to pin it. These triggers provide a way of updating elements of the nested table. They fire for each nested table element being modified.
The row correlation variables inside the trigger correspond to the nested table element. This type of trigger also provides an additional correlation name for accessing the parent row that contains the nested table being modified.
Can only be defined over nested table columns in views. They do not fire when a DML statement is performed on the view. For example, consider a department view that contains a nested table of employees. Now, if you want to modify the emplist column, which is the nested table of employees, then you can define an INSTEAD OF trigger over the column to handle the operation. The following example shows how an insert trigger might be written: For example, you define the following trigger: You may need to set up the following data structures for certain examples to work: For the order of trigger firing, see Oracle9i Database Concepts.
The statement level triggers are useful for performing validation checks for the entire statement. A WHEN clause cannot be included in the definition of a statement trigger.
If included, then the expression in the WHEN clause is evaluated for each row that the trigger affects. If the expression evaluates to TRUE for a row, then the trigger body is fired on behalf of that row. In more realistic examples, you might test if one column value is less than another.
The expression in a WHEN clause of a row trigger can include correlation names, which are explained later. These statements are run if the triggering statement is entered and if the trigger restriction if included evaluates to TRUE.
Monitoring Logons with a Trigger Note: You may need to set up data structures similar to the following for certain examples to work: Ename The corresponding Java file is thjvTriggers.
Two correlation names exist for every column of the table being modified: Depending on the type of triggering statement, certain correlation names might not have any meaning. Because the row no longer exists after the row is deleted, the: However, you cannot modify: ORA is raised if you try to modify: The new column values are referenced using the new qualifier before the column name, while the old column values are referenced using the old qualifier before the column name.
The parent row corresponding to this nested table element can be accessed using the parent qualifier. The parent correlation name is meaningful and valid only inside a nested table trigger.
Because this is rare, this option is infrequently used. For example, assume you have a table named new with columns field1 number and field2 character.
Within the code of the trigger body, you can execute blocks of code depending on the kind of DML operation fired the trigger: For example, assume a trigger is defined as the following: This way, the trigger can minimize its overhead when the column of interest is not being changed. Error Conditions and Exceptions in the Trigger Body If a predefined or user-defined error condition or exception is raised during the execution of a trigger body, then all effects of the trigger body, as well as the triggering statement, are rolled back unless the error is trapped by an exception handler.
Therefore, a trigger body can prevent the execution of the triggering statement by raising an exception. User-defined exceptions are commonly used in triggers that enforce complex security authorizations or integrity constraints.
In these scenarios, only the trigger action is rolled back. Triggers and Handling Remote Exceptions A trigger that accesses a remote site cannot do remote exception handling if the network link is unavailable. Thus, if a remote site is unavailable when the trigger must compile, then Oracle cannot validate the statement accessing the remote database, and the compilation fails.
The previous example exception statement cannot run, because the trigger does not complete compilation. Because stored procedures are stored in a compiled form, the work-around for the previous example is as follows: The following sections discuss these restrictions.
Maximum Trigger Size The size of a trigger cannot be more than 32K. DDL statements are not allowed in the body of a trigger. Also, no transaction control statements are allowed in a trigger. A procedure called by a trigger cannot run the previous transaction control statements, because the procedure runs within the context of the trigger body. Statements inside a trigger can reference remote schema objects.
However, pay special attention when calling remote procedures from within a local trigger. If a timestamp or signature mismatch is found during execution of the trigger, then the remote procedure is not run, and the trigger is invalidated. The maximum length for these datatypes is bytes. This can occur many times before the statement completes successfully. The rollback to savepoint does not undo changes to any package variables referenced in the trigger.
The package should include a counter variable to detect this situation. Therefore, do not create triggers that depend on the order in which rows are processed. For example, do not assign a value to a global package variable in a row trigger if the current value of the global variable is dependent on the row being processed by the row trigger.
Also, if global package variables are updated within a trigger, then it is best to initialize those variables in a BEFORE statement trigger. When a statement in a trigger body causes another trigger to be fired, the triggers are said to be cascading.
Oracle allows up to 32 triggers to cascade at any one time. Trigger Evaluation Order Although any trigger can run a sequence of operations either in-line or by calling procedures, using multiple triggers of the same type enhances database administration by permitting the modular installation of applications that have triggers on the same tables.
Oracle executes all triggers of the same type before executing triggers of a different type. If you have multiple triggers of the same type on a single table, then Oracle chooses an arbitrary order to execute these triggers.
Oracle9i Database Concepts has more information on the firing order of triggers. Each subsequent trigger sees the changes made by the previously fired triggers. Each trigger can see the old and new values.
To ensure that multiple triggered actions occur in a specific order, you must consolidate these actions into a single trigger for example, by having the trigger call a series of procedures. You cannot open a database that contains multiple triggers of the same type if you are using any version of Oracle before release 7.
For system triggers, compatibility must be 8. The session that issued the triggering statement cannot query or modify a mutating table. This restriction prevents a trigger from seeing an inconsistent set of data. When a trigger encounters a mutating table, a runtime error occurs, the effects of the trigger body and triggering statement are rolled back, and control is returned to the user or application. Consider the following trigger: For example, in place of a single AFTER row trigger that updates the original table, resulting in a mutating table error, you might use two triggers--an AFTER row trigger that updates a temporary table, and an AFTER statement trigger that updates the original table with the values from the temporary table.
Declarative integrity constraints are checked at various times with respect to row triggers. Oracle9i Database Concepts has information about the interaction of triggers and integrity constraints. Because declarative referential integrity constraints are currently not supported between tables on different nodes of a distributed database, the mutating table restrictions do not apply to triggers that access remote nodes.
These restrictions are also not enforced among tables in the same database that are connected by loop-back database links. A loop-back database link makes a local table appear remote by defining an Oracle Net path back to the database that contains the link.
Do not use loop-back database links to circumvent the trigger restrictions. Such applications might behave unpredictably. Restrictions on Mutating Tables Relaxed Before Oracle8i, there was a "constraining error" that prevented a row trigger from modifying a table when the parent statement implicitly read that table to enforce a foreign key constraint.
Starting with Oracle8i, there is no constraining error. Also, checking of the foreign key is deferred until at least the end of the parent statement. The mutating error still prevents the trigger from reading or modifying the table that the parent statement is modifying. However, starting in Oracle release 8. That way, you can create triggers just not row triggers to read and modify the parent and child tables. This allows most foreign key constraint actions to be implemented through their obvious after-row trigger, providing the constraint is not self-referential.
Update cascade, update set null, update set default, delete set default, inserting a missing parent, and maintaining a count of children can all be implemented easily. For example, this is an implementation of update cascade: For example, if a table p has three rows with the values 1 , 2 , 3 , and table f also has three rows with the values 1 , 2 , 3 , then the following statement updates p correctly but causes problems when the trigger updates f: Then the statement updates 2 to 3 in p, and the trigger updates both rows of value 2 to 3 in f.
Finally, the statement updates 3 to 4 in p, and the trigger updates all three rows in f from 3 to 4. The relationship of the data in p and f is lost. To avoid this problem, you must forbid multirow updates to p that change the primary key and reuse existing primary key values. It could also be solved by tracking which foreign key values have already been updated, then modifying the trigger so that no row is updated twice.