Publish information about database events, user events, and SQL statements to subscribing applications. Enforce complex business or referential integrity rules that you cannot define with constraints see "How Triggers and Constraints Differ". Both triggers and constraints can constrain data input, but they differ significantly.
A trigger always applies to new data only. 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.
A constraint can apply either to new data only like a trigger or to both new and existing data. Constraints are easier to write and less error-prone than triggers that enforce the same rules.
However, triggers can enforce some complex business rules that constraints cannot. Oracle strongly recommends that you use triggers to constrain data input only in these situations:. To enforce referential integrity when child and parent tables are on different nodes of a distributed database. To enforce complex business or referential integrity rules that you cannot define with constraints.
Oracle Database Advanced Application Developer's Guide for information about using constraints to enforce business rules and prevent the entry of invalid information into tables.
A simple DML trigger fires at exactly one of these timing points:. A compound DML trigger created on a table or editioning view can fire at one, some, or all of the preceding timing points. Compound DML triggers help program an approach where you want the actions that you implement for the various timing points to share common data. A simple or compound DML trigger that fires at row level can access the data in the row that it is processing.
For details, see "Correlation Names and Pseudorecords". A crossedition trigger is a simple or compound DML trigger for use only in edition-based redefinition. With a column list, the trigger fires only when a specified column is updated.
Without a column list, the trigger fires when any column of the associated table is updated. The triggering event of a DML trigger can be composed of multiple triggering statements. When one of them fires the trigger, the trigger can determine which one by using these conditional predicates :.
Example creates a DML trigger that uses conditional predicates to determine which of its four possible triggering statements fired it. A trigger that fires at row level can access the data in the row that it is processing by using correlation names. Reference the field of a pseudorecord with this syntax:. In the WHEN clause of a conditional trigger, a correlation name is not a placeholder for a bind variable. Therefore, omit the colon in the preceding syntax. The database evaluates the WHEN condition for each affected row.
Example creates a trigger that modifies CLOB columns. Example creates a table with the same name as a correlation name, new , and then creates a trigger on that table. To avoid conflict between the table name and the correlation name, the trigger references the correlation name as Newest. The old and new values are : OLD.
All values of column n were increased by 1. The value of m remains 0. Example creates the view oe. The trigger inserts rows into the base tables of the view, customers and orders. The trigger fires for each modified nested table element. A compound DML trigger created on a table or editioning view can fire at multiple timing points.
The common state is established when the triggering statement starts and is destroyed when the triggering statement completes, even when the triggering statement causes an error. A compound DML trigger created on a noneditioning view is not really compound, because it has only one timing point section. The optional declarative part of a compound trigger declares variables and subprograms that all of its timing-point sections can use.
When the trigger fires, the declarative part runs before any timing-point sections run. The variables and subprograms exist for the duration of the triggering statement. The syntax for creating the simplest compound DML trigger on a noneditioning view is:. A compound DML trigger created on a table or editioning view has at least one timing-point section in Table If the trigger has multiple timing-point sections, they can be in any order, but no timing-point section can be repeated.
If a timing-point section is absent, then nothing happens at its timing point. A compound DML trigger has a performance benefit when the triggering statement affects many rows. For example, suppose that this statement triggers a compound DML trigger that has all four timing-point sections in Table :.
A compound DML trigger is useful for accumulating rows destined for a second table so that you can periodically bulk-insert them. Scenario: You want to log every change to hr. Solution: Define a compound trigger on updates of the table hr. This rule must be enforced by a trigger. The state variables are initialized each time the trigger fires even when the triggering statement is interrupted and restarted. You can use triggers and constraints to maintain referential integrity between parent and child tables, as Table shows.
No action is required for inserts into the parent table, because no dependent foreign keys exist. The database does not support declarative referential constraints between tables on different nodes of a distributed database. This operation is necessary to maintain concurrency while the rows are being processed. These examples are not meant to be used exactly as written. They are provided to assist you in designing your own triggers. The triggers in Example ensure that if a department number is updated in the dept table, then this change is propagated to dependent foreign keys in the emp table.
Triggers can enforce integrity rules other than referential integrity. The trigger in Example does a complex check before allowing the triggering statement to run. Triggers are commonly used to enforce complex security authorizations for table data. Use triggers only to enforce complex security authorizations that you cannot define using the database security features provided with the database.
For example, use a trigger to prohibit updates to the employee table during weekends and nonworking hours. The security check is done before the triggering statement is allowed to run, so that no wasted work is done by an unauthorized statement. The security check is done only for the triggering statement, not for each row affected by the triggering statement.
The trigger in Example enforces security by raising exceptions when anyone tries to update the table employees during weekends or nonworking hours. Example Trigger Enforces Security Authorizations. Triggers are very useful when you want to transparently do a related change in the database following certain events. This type of trigger is useful to force values in specific columns that depend on the values of other columns in the same row.
The trigger in Example derives new column values for a table whenever a row is inserted or updated. Views are an excellent mechanism to provide logical windows over table data. However, when the view query gets complex, the system implicitly cannot translate the DML on the view into those on the underlying tables.
These triggers can be defined over views, and they fire instead of the actual DML. Consider a library system where books are arranged by title. The library consists of a collection of book type objects:.
Similarly, you can also define triggers on the nested table booklist to handle modification of the nested table element. An application context captures session-related information about the user who is logging in to the database. From there, your application can control how much access this user has, based on his or her session information.
A system trigger is created on either a schema or the database. A SCHEMA trigger is created on a schema and fires whenever the user who owns it is the current user and initiates the triggering event. Suppose that both user1 and user2 own schema triggers, and user1 invokes a DR unit owned by user2. Inside the DR unit, user2 is the current user.
Therefore, if the DR unit initiates the triggering event of a schema trigger that user2 owns, then that trigger fires. However, if the DR unit initiates the triggering event of a schema trigger that user1 owns, then that trigger does not fire.
When a user connected as HR tries to drop a database object, the database fires the trigger before dropping the object. Example shows the basic syntax for a trigger to log errors. This trigger fires after an unsuccessful statement execution, such as unsuccessful logon. The database fires the trigger instead of executing its triggering statement.
The trigger in Example invokes a Java subprogram. A subprogram invoked by a trigger cannot run transaction control statements, because the subprogram runs in the context of the trigger body. If a trigger invokes an invoker rights IR subprogram, then the user who created the trigger, not the user who ran the triggering statement, is considered to be the current user.
If a trigger invokes a remote subprogram, and a time stamp or signature mismatch is found during execution of the trigger, then the remote subprogram does not run and the trigger is invalidated.
If a compilation error occurs, the trigger is still created, but its triggering statement fails, except in these cases:. If a trigger does not compile successfully, then its exception handler cannot run. For an example, see "Remote Exception Handling". If a trigger references another object, such as a subprogram or package, and that object is modified or dropped, then the trigger becomes invalid.
The next time the triggering event occurs, the compiler tries to revalidate the trigger for details, see Oracle Database Advanced Application Developer's Guide. In most cases, if a trigger runs a statement that raises an exception, and the exception is not handled by an exception handler, then the database rolls back the effects of both the trigger and its triggering statement. In the following cases, the database rolls back only the effects of the trigger, not the effects of the triggering statement and logs the error in trace files and the alert log :.
In the case of a compound DML trigger, the database rolls back only the effects of the triggering statement, not the effects of the trigger. However, variables declared in the trigger are re-initialized, and any values computed before the triggering statement was rolled back are lost. A trigger that accesses a remote database can do remote exception handling only if the remote database is available.
If the remote database is unavailable when the local database must compile the trigger, then the local database cannot validate the statement that accesses the remote database, and the compilation fails. If the trigger cannot be compiled, then its exception handler cannot run. The trigger also has an exception handler. However, if the remote database is unavailable when the local database tries to compile the trigger, then the compilation fails and the exception handler cannot run.
Example shows the workaround for the problem in Example : Put the remote INSERT statement and exception handler in a stored subprogram and have the trigger invoke the stored subprogram. The subprogram is stored in the local database in compiled form, with a validated statement for accessing the remote database. Therefore, when the remote INSERT statement fails because the remote database is unavailable, the exception handler in the subprogram can handle it.
Example Workaround for Example Use triggers to ensure that whenever a specific event occurs, any necessary actions are done regardless of which user or application issues the triggering statement. For example, use a trigger to ensure that whenever anyone updates a table, its log file is updated. For example, do not create a trigger to reject invalid data if you can do the same with constraints see "How Triggers and Constraints Differ".
Do not create triggers that depend on the order in which a SQL statement processes rows which can vary. For example, do not assign a value to a global package variable in a row trigger if the current value of the variable depends on the row being processed by the row trigger. The database can do this many times before the triggering statement completes successfully.
Each time the database restarts the triggering statement, the trigger fires. To detect this situation, include a counter variable in the package. The trigger fires recursively until it runs out of memory. If you create a trigger that includes a statement that accesses a remote database, then put the exception handler for that statement in a stored subprogram and invoke the subprogram from the trigger.
They fire every time any database user initiates a triggering event. If a trigger runs the following statement, the statement returns the owner of the trigger, not the user who is updating the table:. Therefore, the following statement cannot fire the trigger that it creates:. To allow the modular installation of applications that have triggers on the same tables, create multiple triggers of the same type, rather than a single trigger that runs a sequence of operations.
Each trigger sees the changes made by the previously fired triggers. For information about autonomous triggers, see "Autonomous Triggers". A trigger cannot invoke a subprogram that runs transaction control statements, because the subprogram runs in the context of the trigger body. For more information about subprograms invoked by triggers, see "Subprograms Invoked by Triggers". For information about subprograms invoked by triggers, see "Subprograms Invoked by Triggers".
The mutating-table restriction prevents the trigger from querying or modifying the table that the triggering statement is modifying. When a row-level trigger encounters a mutating table, ORA occurs, the effects of the trigger and triggering statement are rolled back, and control returns to the user or application that issued the triggering statement, as Example shows. Similarly, the database does not enforce the mutating-table restriction for 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. If you must use a trigger to update a mutating table, you can avoid the mutating-table error in either of these ways:. For example, instead of using one AFTER each row trigger that updates the mutating table, use two triggers—an AFTER each row trigger that updates the temporary table and an AFTER statement trigger that updates the mutating table with the values from the temporary table.
Therefore, you can create row-level and statement-level triggers that query and modify the parent and child tables. This allows most foreign key constraint actions to be implemented through their after-row triggers unless the constraint is 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—see "Triggers for Ensuring Referential Integrity".
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