Designing Triggers
Use the following guidelines when designing triggers:
- Use triggers to guarantee that when a specific operation is performed, related actions are performed.
- Use database triggers only for centralized, global operations that should be fired for the triggering statement, regardless of which user or database application issues the statement.
- Do not define triggers that duplicate the functionality already built into Oracle. For example, do not define triggers to enforce data integrity rules that can be easily enforced using declarative integrity constraints.
- Limit the size of triggers (60 lines or fewer is a good guideline). If the logic for your trigger requires much more than 60 lines of PL/SQL code, it is better to include most of the code in a stored procedure, and call the procedure from the trigger.
- Be careful not to create recursive triggers. For example, creating an AFTER UPDATE statement trigger on the EMP table that itself issues an UPDATE statement on EMP causes the trigger to fire recursively until it has run out of memory.
Creating Triggers
Triggers are created using the CREATE TRIGGER command. This command can be used with any interactive tool, such as SQL*Plus or Enterprise Manager. When using an interactive tool, a solitary slash ( / ) on the last line is used to activate the CREATE TRIGGER statement.
The following statement creates a trigger for the EMP table:
CREATE TRIGGER print_salary_changes BEFORE DELETE OR INSERT OR UPDATE ON emp FOR EACH ROW WHEN (new.empno > 0) DECLARE sal_diff number; BEGIN sal_diff := new.sal - old.sal; dbms_output.put('Old salary: ' || :old.sal); dbms_output.put(' New salary: ' || :new.sal); dbms_output.put_line(' Difference ' || sal_diff); END; /
If you enter a SQL statement such as
UPDATE emp SET sal = sal + 500.00 WHERE deptno = 10
the trigger will fire once for each row that is updated, and it prints the new and old salaries, and the difference.
The CREATE (or CREATE OR REPLACE) statement fails if any errors exist in the PL/SQL block.
The following sections use this example to illustrate the way that parts of a trigger are specified. For more realistic examples of CREATE TRIGGER statements, see "Examples of Trigger Applications" on page 13-22.
Prerequisites for Creating Triggers
Before creating any triggers, while connected as SYS, submit the CATPROC.SQL script. This script automatically runs all of the scripts required for, or used within, the procedural extensions to the Oracle Server.
Note:
The location of this file is operating system dependent; see your platform-specific Oracle documentation.
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Naming Triggers
Trigger names must be unique with respect to other triggers in the same schema. Trigger names do not have to be unique with respect to other schema objects such as tables, views, and procedures. For example, a table and a trigger can have the same name (although, to avoid confusion, this is not recommended).
The BEFORE and AFTER Options
The BEFORE or AFTER option in the CREATE TRIGGER statement specifies exactly when to file the trigger body in relation to the triggering statement that is being executed. In a CREATE TRIGGER statement, the BEFORE or AFTER option is specified just before the triggering statement. For example, the PRINT_SALARY_CHANGES trigger in the previous example is a BEFORE trigger.
The INSTEAD OF Option
The INSTEAD OF option in the CREATE TRIGGER statement is an alternative to the BEFORE and AFTER options. INSTEAD OF triggers provide a transparent way of modifying views that cannot be modified directly through UPDATE, INSERT, and DELETE statements. These triggers are called INSTEAD OF triggers because, unlike other types of triggers, Oracle fires the trigger instead of executing the triggering statement. The trigger performs update, insert, or delete operations directly on the underlying tables.
Users write normal UPDATE, INSERT, and DELETE statements against the view, and the INSTEAD OF trigger works invisibly in the background to make the right actions take place.
By default, INSTEAD OF triggers are activated for each row (see "FOR EACH ROW Option" on page 13-7).
Views That Are Not Modifiable
A view cannot be modified by UPDATE, INSERT, or DELETE statements if the view query contains any of the following constructs:
- set operators
- group functions
- GROUP BY, CONNECT BY, or START WITH clauses
- the DISTINCT operator
- joins (a subset of join views are updatable)
If a view contains pseudocolumns or expressions, you can only update the view with an UPDATE statement that does not refer to any of the pseudocolumns or expressions.
Example of an INSTEAD OF Trigger
The following example shows an INSTEAD OF trigger for inserting rows into the MANAGER_INFO view.
CREATE VIEW manager_info AS SELECT e.name, e.empno, d.dept_type, d.deptno, p.level, p.projno FROM emp e, dept d, project p WHERE e.empno = d.mgr_no AND d.deptno = p.resp_dept; CREATE TRIGGER manager_info_insert INSTEAD OF INSERT ON manager_info REFERENCING NEW AS n -- new manager information FOR EACH ROW BEGIN IF NOT EXISTS SELECT * FROM emp WHERE emp.empno = :n.empno THEN INSERT INTO emp VALUES(:n.empno, :n.name); ELSE UPDATE emp SET emp.name = :n.name WHERE emp.empno = :n.empno; END IF; IF NOT EXISTS SELECT * FROM dept WHERE dept.deptno = :n.deptno THEN INSERT INTO dept VALUES(:n.deptno, :n.dept_type); ELSE UPDATE dept SET dept.dept_type = :n.dept_type WHERE dept.deptno = :n.deptno; END IF; IF NOT EXISTS SELECT * FROM project WHERE project.projno = :n.projno THEN INSERT INTO project VALUES(:n.projno, :n.project_level); ELSE UPDATE project SET project.level = :n.level WHERE project.projno = :n.projno; END IF; END;
The actions shown for rows being inserted into the MANAGER_INFO view first test to see if appropriate rows already exist in the base tables from which MANAGER_INFO is derived. The actions then insert new rows or update existing rows, as appropriate. Similar triggers can specify appropriate actions for UPDATE and DELETE.
Object Views and INSTEAD OF Triggers
INSTEAD OF triggers provide the means to modify object view instances on the client-side through OCI calls. (See Oracle Call Interface Programmer's Guide for more information.) To modify an object materialized by an object view in the client-side object cache and flush it back to the persistent store, the user must specify INSTEAD OF triggers, unless the object view is modifiable. If the object is read only, however, it is not necessary to define triggers to pin it.
| Note Also:
For more examples of INSTEAD OF triggers, see "Updating the Object Views" on page 8-6.
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Triggering Statement
The triggering statement specifies:
- the type of SQL statement that fires the trigger body.The possible options include DELETE, INSERT, and UPDATE. One, two, or all three of these options can be included in the triggering statement specification.
- the table associated with the trigger.
For example, the PRINT_SALARY_CHANGES trigger on page 13-2 fires after any DELETE, INSERT, or UPDATE on the EMP table. Any of the following statements would trigger the PRINT_SALARY_CHANGES trigger given in the previous example:
DELETE FROM emp; INSERT INTO emp VALUES ( . . . ); INSERT INTO emp SELECT . . . FROM . . . ; UPDATE emp SET . . . ;
Column List for UPDATE
If a triggering statement specifies UPDATE, an optional list of columns can be included in the triggering statement. If you include a column list, the trigger is fired on an UPDATE statement only when one of the specified columns is updated. If you omit a column list, the trigger is fired when any column of the associated table is updated. A column list cannot be specified for INSERT or DELETE triggering statements.
The previous example of the PRINT_SALARY_CHANGES trigger might have included a column list in the triggering statement, as in
. . . BEFORE DELETE OR INSERT OR UPDATE OF ename ON emp . . .
FOR EACH ROW Option
The FOR EACH ROW option determines whether the trigger is a row trigger or a statement trigger. If you specify FOR EACH ROW, the trigger fires once for each row of the table that is affected by the triggering statement. The absence of the FOR EACH ROW option means that the trigger fires only once for each applicable statement, but not separately for each row affected by the statement.
For example, you define the following trigger:
CREATE TRIGGER log_salary_increase AFTER UPDATE ON emp FOR EACH ROW WHEN (new.sal > 1000) BEGIN INSERT INTO emp_log (emp_id, log_date, new_salary, action) VALUES (:new.empno, SYSDATE, :new.sal, 'NEW SAL'); END;
and then issue the SQL statement:
UPDATE emp SET sal = sal + 1000.0 WHERE deptno = 20;
If there are five employees in department 20, the trigger will fire five times when this statement is issued, since five rows are affected.
The following trigger fires only once for each UPDATE of the EMP table:
CREATE TRIGGER log_emp_update AFTER UPDATE ON emp BEGIN INSERT INTO emp_log (log_date, action) VALUES (SYSDATE, 'EMP COMMISSIONS CHANGED'); END;
| See Also:
For the order of trigger firing, see Oracle8 Concepts manual.
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The WHEN Clause
Optionally, a trigger restriction can be included in the definition of a row trigger by specifying a Boolean SQL expression in a WHEN clause (a WHEN clause cannot be included in the definition of a statement trigger). If included, the expression in the WHEN clause is evaluated for each row that the trigger affects. If the expression evaluates to TRUE for a row, the trigger body is fired on behalf of that row. However, if the expression evaluates to FALSE or NOT TRUE (that is, unknown, as with nulls) for a row, the trigger body is not fired for that row. The evaluation of the WHEN clause does not have an effect on the execution of the triggering SQL statement (that is, the triggering statement is not rolled back if the expression in a WHEN clause evaluates to FALSE).
For example, in the PRINT_SALARY_CHANGES trigger, the trigger body would not be executed if the new value of EMPNO is zero, NULL, or negative. 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 below. The expression in a WHEN clause must be a SQL expression and cannot include a subquery. You cannot use a PL/SQL expression (including user-defined functions) in the WHEN clause.
The Trigger Body
The trigger body is a PL/SQL block that can include SQL and PL/SQL statements. These statements are executed if the triggering statement is issued and the trigger restriction (if included) evaluates to TRUE. The trigger body for row triggers has some special constructs that can be included in the code of the PL/SQL block: correlation names and the REFERENCEING option, and the conditional predicates INSERTING, DELETING, and UPDATING.
Accessing Column Values in Row Triggers
Within a trigger body of a row trigger, the PL/SQL code and SQL statements have access to the old and new column values of the current row affected by the triggering statement. Twocorrelation names exist for every column of the table being modified: one for the old column value and one for the new column value. Depending on the type of triggering statement, certain correlation names might not have any meaning.
- A trigger fired by an INSERT statement has meaningful access to new column values only. Because the row is being created by the INSERT, the old values are null.
- A trigger fired by an UPDATE statement has access to both old and new column values for both BEFORE and AFTER row triggers.
- A trigger fired by a DELETE statement has meaningful access to old column values only. Because the row will no longer exist after the row is deleted, the new values are null.
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. For example, if the triggering statement is associated with the EMP table (with the columns SAL, COMM, etc.), you can include statements in the trigger body similar to
IF :new.sal > 10000 . . . IF :new.sal < :old.sal . . .
Old and new values are available in both BEFORE and AFTER row triggers. A NEW column value can be assigned in a BEFORE row trigger, but not in an AFTER row trigger (because the triggering statement takes effect before an AFTER row trigger is fired). If a BEFORE row trigger changes the value of NEW.COLUMN, an AFTER row trigger fired by the same statement sees the change assigned by the BEFORE row trigger.
Correlation names can also be used in the Boolean expression of a WHEN clause. A colon must precede the OLD and NEW qualifiers when they are used in a trigger's body, but a colon is not allowed when using the qualifiers in the WHEN clause or the REFERENCING option.
The REFERENCING Option
The REFERENCING option can be specified in a trigger body of a row trigger to avoid name conflicts among the correlation names and tables that might be named "OLD" or "NEW". Since this is rare, this option is infrequently used.
For example, assume you have a table named NEW with columns FIELD1 (number) and FIELD2 (character). The following CREATE TRIGGER example shows a trigger associated with the NEW table that can use correlation names and avoid naming conflicts between the correlation names and the table name:
CREATE TRIGGER PRINT_SALARY_CHANGES BEFORE UPDATE ON new REFERENCING new AS newest FOR EACH ROW BEGIN :newest.field2 := TO_CHAR (:newest.field1); END;
Notice that the NEW qualifier is renamed to NEWEST using the REFERENCING option, and is then used in the trigger body.
Conditional Predicates
If more than one type of DML operation can fire a trigger (for example, "ON INSERT OR DELETE OR UPDATE OF emp"), the trigger body can use the conditional predicates INSERTING, DELETING, and UPDATING to execute specific blocks of code, depending on the type of statement that fires the trigger. Assume this is the triggering statement:
INSERT OR UPDATE ON emp
Within the code of the trigger body, you can include the following conditions:
IF INSERTING THEN . . . END IF; IF UPDATING THEN . . . END IF;
The first condition evaluates to TRUE only if the statement that fired the trigger is an INSERT statement; the second condition evaluates to TRUE only if the statement that fired the trigger is an UPDATE statement.
In an UPDATE trigger, a column name can be specified with an UPDATING conditional predicate to determine if the named column is being updated. For example, assume a trigger is defined as
CREATE TRIGGER . . . . . . UPDATE OF sal, comm ON emp . . . BEGIN . . . IF UPDATING ('SAL') THEN . . . END IF; END;
The code in the THEN clause executes only if the triggering UPDATE statement updates the SAL column. The following statement would fire the above trigger and cause the UPDATING (sal) conditional predicate to evaluate to TRUE:
UPDATE emp SET sal = sal + 100;
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, 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.
| See Also:
For more information about error processing in PL/SQL program units, see "Error Handling" on page 10-30 and "Declaring Exceptions and Exception Handling Routines" on page 10-32.
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Triggers and Handling Remote Exceptions
A trigger that accesses a remote site cannot do remote exception handling if the network link is unavailable. For example:
CREATE TRIGGER example AFTER INSERT ON emp FOR EACH ROW BEGIN INSERT INTO emp@remote -- <- compilation fails here VALUES ('x'); -- when dblink is inaccessible EXCEPTION WHEN OTHERS THEN INSERT INTO emp_log VALUES ('x'); END;
A trigger is compiled when it is created. Thus, if a remote site is unavailable when the trigger must compile, Oracle cannot validate the statement accessing the remote database, and the compilation fails. The previous example exception statement cannot execute because the trigger does not complete compilation.
Because stored procedures are stored in a compiled form, the work-around for the above example is as follows:
CREATE TRIGGER example AFTER INSERT ON emp FOR EACH ROW BEGIN insert_row_proc; END; CREATE PROCEDURE insert_row_proc BEGIN INSERT INTO emp@remote VALUES ('x'); EXCEPTION WHEN OTHERS THEN INSERT INTO emp_log VALUES ('x'); END;
The trigger in this example compiles successfully and calls the stored procedure, which already has a validated statement for accessing the remote database; thus, when the remote INSERT statement fails because the link is down, the exception is caught.
Restrictions on Creating Triggers
Coding a trigger requires some restrictions that are not required for standard PL/SQL blocks. The following sections discuss these restrictions.
Valid SQL Statements in Trigger Bodies
The body of a trigger can contain DML SQL statements. It can also contain SELECT statements, but they must be SELECT... INTO... statements or the SELECT statement in the definition of a cursor).
DDL statements are not allowed in the body of a trigger. Also, no transaction control statements are allowed in a trigger. The commands ROLLBACK, COMMIT, and SAVEPOINT cannot be used.
Note:
A procedure called by a trigger cannot execute the above transaction control statements because the procedure executes within the context of the trigger body.
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Statements inside a trigger can reference remote schema objects. However, pay special attention when calling remote procedures from within a local trigger; since if a timestamp or signature mismatch is found during execution of the trigger, the remote procedure is not executed and the trigger is invalidated.
LONG and LONG RAW Datatypes
LONG and LONG RAW datatypes in triggers are subject to the following restrictions:- A SQL statement within a trigger can insert data into a column of
LONGorLONGRAWdatatype. - If data from a
LONGorLONGRAWcolumn can be converted to a constrained datatype (such asCHARandVARCHAR2), aLONGorLONGRAWcolumn can be referenced in a SQL statement within a trigger. Note that the maximum length for these datatypes is 32000 bytes. - Variables cannot be declared using the
LONGorLONGRAWdatatypes. - :
NEWand :OLDcannot be used withLONGorLONGRAWcolumns.
References to Package Variables
If an
UPDATE or DELETE statement detects a conflict with a concurrent UPDATE, Oracle performs a transparent rollback to savepoint and restarts the update. This can occur many times before the statement completes successfully. Each time the statement is restarted, the BEFORE STATEMENT trigger is fired again. 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.Row Evaluation Order
A relational database does not guarantee the order of rows processed by a SQL statement. Therefore, do not create triggers that depend on the order in which rows will be 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, 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. However, you can effectively limit the number of trigger cascades using the initialization parameter
OPEN_CURSORS, because a cursor must be opened for every execution of a trigger.Trigger Evaluation Order
Although any trigger can execute 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, Oracle chooses an arbitrary order to execute these triggers.
| See Also:
Oracle8 Concepts manual for more information on the firing order of triggers.
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Each subsequent trigger sees the changes made by the previously fired triggers. Each trigger can see the old and new values. The old values are the original values and the new values are the current values as set by the most recently fired
UPDATE or INSERT trigger.
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 7.1, nor can you open such a database if your
COMPATIBLEinitialization parameter is set to a version earlier than 7.1.0.Mutating and Constraining Tables
A mutating table is a table that is currently being modified by an
UPDATE, DELETE, or INSERT statement, or a table that might need to be updated by the effects of a declarative DELETE CASCADEreferential integrity constraint. A constraining table is a table that a triggering statement might need to read either directly, for a SQL statement, or indirectly, for a declarative referential integrity constraint. A table is mutating or constraining only to the session that issued the statement in progress.
Tables are never considered mutating or constraining for statement triggers unless the trigger is fired as the result of a
DELETE CASCADE.
For all row triggers, or for statement triggers that were fired as the result of a
DELETE CASCADE, there are two important restrictions regarding mutating and constraining tables. These restrictions prevent a trigger from seeing an inconsistent set of data.- The SQL statements of a trigger cannot read from (query) or modify a mutating table of the triggering statement.
- The statements of a trigger cannot change the
PRIMARY,FOREIGN, orUNIQUEKEYcolumns of a constraining table of the triggering statement.There is an exception to this restriction;BEFOREROWandAFTERROWtriggers fired by a single rowINSERTto a table do not treat that table as mutating or constraining. Note thatINSERTstatements that may involve more than one row, such asINSERTINTOempSELECT. . ., are not considered single row inserts, even if they only result in one row being inserted.
Figure 13-1 illustrates the restriction placed on mutating tables.
Figure 13-1 Mutating Tables
Notice that the SQL statement is executed for the first row of the table and then an
AFTER ROW trigger is fired. In turn, a statement in the AFTER ROW trigger body attempts to query the original table. However, because the EMP table is mutating, this query is not allowed by Oracle. If attempted, 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:
CREATE OR REPLACE TRIGGER emp_count AFTER DELETE ON EMP FOR EACH ROW DECLARE n INTEGER; BEGIN SELECT COUNT(*) INTO n FROM emp; DBMS_OUTPUT.PUT_LINE(' There are now ' || n || ' employees.'); END;
If the SQL statement
DELETE FROM emp WHERE empno = 7499;
is issued, the following error is returned:
ORA-04091: table SCOTT.EMP is mutating, trigger/function may not see it
Oracle returns this error when the trigger fires since the table is mutating when the first row is deleted. (Only one row is deleted by the statement, since
EMPNO is a primary key, but Oracle has no way of knowing that.)
If you delete the line "
FOR EACH ROW" from the trigger above, the trigger becomes a statement trigger, the table is not mutating when the trigger fires, and the trigger does output the correct data.
If you need to update a mutating or constraining table, you could use a temporary table, a PL/SQL table, or a package variable to bypass these restrictions. For example, in place of a single
AFTERrow trigger that updates the original table, resulting in a mutating table error, you may be able to 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.
| See Also:
Oracle8 Concepts for information about the interaction of triggers and integrity constraints.
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Because declarative referential integrity constraints are currently not supported between tables on different nodes of a distributed database, the constraining 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 a Net8 path back to the database that contains the link.
You should not use loop-back database links to circumvent the trigger restrictions. Such applications might behave unpredictably.
Who Is the Trigger User?
If you issue the statement
SELECT username FROM USER_USERS
in a trigger, the name of the owner of the trigger is returned, not the name of user who is updating the table.
Privileges Required to Create Triggers
To create a trigger in your schema, you must have the
CREATE TRIGGER system privilege, and either- own the table specified in the triggering statement, or
- have the
ALTERprivilege for the table in the triggering statement, or - have the
ALTERANYTABLEsystem privilege
To create a trigger in another user's schema, you must have the
CREATE ANY TRIGGER system privilege. With this privilege, the trigger can be created in any schema and can be associated with any user's table.Privileges for Referenced Schema Objects
The object privileges to the schema objects referenced in the trigger body must be granted to the trigger's owner explicitly (not via a role). The statements in the trigger body operate under the privilege domain of the trigger's owner, not the privilege domain of the user issuing the triggering statement. This is similar to stored procedures.
When Triggers Are Compiled
Triggers are similar to PL/SQL anonymous blocks with the addition of the :
NEW and :OLD capabilities, but their compilation is different. A PL/SQL anonymous block is compiled each time it is loaded into memory. Compilation involves three stages:- syntax checking: PL/SQL syntax is checked and a parse tree is generated
- semantic checking: type checking and further processing on the parse tree
- code generation: the pcode is generated
Triggers, in contrast, are fully compiled when the
CREATE TRIGGER command is issued, and the pcode is stored in the data dictionary. Hence, firing the trigger no longer requires the opening of a shared cursor to run the trigger action. Instead, the trigger is executed directly.
If errors occur during the compilation of a trigger, the trigger is still created. If a DML statement fires this trigger, the DML statement will fail. (Runtime trigger errors always cause the DML statement to fail.) You can use the
SHOW ERRORS command in SQL*Plus or Enterprise Manager to see any compilation errors when you create a trigger, or you can SELECT the errors from theUSER_ERRORS view.Dependencies
Compiled triggers have dependencies. They become invalid if a depended-on object, such as a stored procedure or a function called from the trigger body, is modified. Triggers that are invalidated for dependency reasons are recompiled when next invoked.
You can examine the
ALL_DEPENDENCIES view to see the dependencies for a trigger. For example, the statementSELECT NAME, REFERENCED_OWNER, REFERENCED_NAME, REFERENCED_TYPE FROM ALL_DEPENDENCIES WHERE OWNER = 'SCOTT' and TYPE = 'TRIGGER';
shows the dependencies for the triggers in the
SCOTT schema.Recompiling a Trigger
Use the
ALTER TRIGGER command to recompile a trigger manually. For example, the commandALTER TRIGGER print_salary_changes COMPILE;
recompiles the
PRINT_SALARY_CHANGES trigger.
To recompile a trigger, you must own the trigger or have the
ALTER ANY TRIGGER system privilege.Migration Issues
Non-compiled triggers cannot be fired under compiled trigger releases (such as Oracle 7.3 and Oracle8). If upgrading from a non-compiled trigger release to a compiled trigger release, all existing triggers must be compiled. The upgrade script cat73xx.sql invalidates all triggers so that they are automatically recompiled when first executed. (The xx stands for a variable minor release number.)
Downgrading from Oracle 7.3 or later to a release prior to 7.3 requires that you execute the cat73xxd.sql downgrade script. This handles portability issues between stored and non-stored trigger releases.
Debugging a Trigger
You can debug a trigger using the same facilities available for stored procedures.
Modifying a Trigger
Like a stored procedure, a trigger cannot be explicitly altered; it must be replaced with a new definition. (The
ALTER TRIGGER command is used only to recompile, enable or disable a trigger.).
When replacing a trigger, you must include the
OR REPLACE option in the CREATE TRIGGER statement. The OR REPLACE option is provided to allow a new version of an existing trigger to replace the older version without affecting any grants made for the original version of the trigger.
Alternatively, the trigger can be dropped using the
DROP TRIGGER command, and you can rerun the CREATE TRIGGER command.
To drop a trigger, the trigger must be in your schema or you must have the
DROP ANY TRIGGER system privilege.Enabling and Disabling Triggers
A trigger can be in one of two distinct modes:
Disabling Triggers
You might temporarily disable a trigger if
- an object it references is not available
- you have to perform a large data load and want it to proceed quickly without firing triggers
- you are reloading data
By default, triggers are enabled when first created. Disable a trigger using the
ALTER TRIGGER command with the DISABLE option. For example, to disable the trigger named REORDER of theINVENTORY table, enter the following statement:ALTER TRIGGER reorder DISABLE;
All triggers associated with a table can be disabled with one statement using the
ALTER TABLE command with the DISABLE clause and the ALL TRIGGERS option. For example, to disable all triggers defined for the INVENTORY table, enter the following statement:ALTER TABLE inventory DISABLE ALL TRIGGERS;
Enabling Triggers
By default, a trigger is automatically enabled when it is created; however, it can later be disabled. Once you have completed the task that required the trigger to be disabled, re-enable the trigger so that it fires when appropriate.
Enable a disabled trigger using the
ALTER TRIGGER command with the ENABLE option. To enable the disabled trigger named REORDER of the INVENTORY table, enter the following statement:ALTER TRIGGER reorder ENABLE;
All triggers defined for a specific table can be enabled with one statement using the
ALTER TABLE command with the ENABLE clause with the ALL TRIGGERS option. For example, to enable all triggers defined for the INVENTORY table, enter the following statement:ALTER TABLE inventory ENABLE ALL TRIGGERS;
Privileges Required to Enable and Disable Triggers
To enable or disable triggers using the
ALTER TABLE command, you must either own the table, have the ALTER schema object privilege for the table, or have the ALTER ANY TABLE system privilege.
To enable or disable triggers using the
ALTER TRIGGER command, you must own the trigger or have the ALTER ANY TRIGGER system privilege.Listing Information About Triggers
The following data dictionary views reveal information about triggers:
The Oracle8 Reference gives a complete description of these data dictionary views. For example, assume the following statement was used to create the
REORDER trigger:CREATE TRIGGER reorder AFTER UPDATE OF parts_on_hand ON inventory FOR EACH ROW WHEN(new.parts_on_hand < new.reorder_point) DECLARE x NUMBER; BEGIN SELECT COUNT(*) INTO x FROM pending_orders WHERE part_no = :new.part_no; IF x = 0 THEN INSERT INTO pending_orders VALUES (:new.part_no, :new.reorder_quantity, sysdate); END IF; END;
The following two queries return information about the
REORDER trigger:SELECT trigger_type, triggering_event, table_name FROM user_triggers WHERE name = 'REORDER'; TYPE TRIGGERING_STATEMENT TABLE_NAME ---------------- -------------------------- ------------ AFTER EACH ROW UPDATE INVENTORY SELECT trigger_body FROM user_triggers WHERE name = 'REORDER'; TRIGGER_BODY -------------------------------------------- DECLARE x NUMBER; BEGIN SELECT COUNT(*) INTO x FROM pending_orders WHERE part_no = :new.part_no; IF x = 0 THEN INSERT INTO pending_orders VALUES (:new.part_no, :new.reorder_quantity, sysdate); END IF; END;
Examples of Trigger Applications
You can use triggers in a number of ways to customize information management in an Oracle database. For example, triggers are commonly used to
- provide sophisticated auditing
- prevent invalid transactions
- enforce referential integrity (either those actions not supported by declarative integrity constraints or across nodes in a distributed database)
- enforce complex business rules
- enforce complex security authorizations
- provide transparent event logging
- automatically generate derived column values
This section provides an example of each of the above trigger applications. These examples are not meant to be used as is, but are provided to assist you in designing your own triggers.
Auditing with Triggers
Triggers are commonly used to supplement the built-in auditing features of Oracle. Although triggers can be written to record information similar to that recorded by the
AUDIT command, triggers should be used only when more detailed audit information is required. For example, use triggers to provide value-based auditing on a per-row basis tables.
Sometimes, the Oracle
AUDIT command is considered a security audit facility, while triggers can provide financial audit facility.
When deciding whether to create a trigger to audit database activity, consider what Oracle's auditing features provide, compared to auditing defined by triggers.
When using triggers to provide sophisticated auditing,
AFTER triggers are normally used. By using AFTER triggers, auditing information is recorded after the triggering statement is subjected to any applicable integrity constraints, preventing cases where the audit processing is carried out unnecessarily for statements that generate exceptions to integrity constraints.
When to use
AFTER row vs. AFTER statement triggers depends on the information being audited. For example, row triggers provide value-based auditing on a per-row basis for tables. Triggers can also require the user to supply a "reason code" for issuing the audited SQL statement, which can be useful in both row and statement-level auditing situations.
The following example demonstrates a trigger that audits modifications to the
EMP table on a per-row basis. It requires that a "reason code" be stored in a global package variable before the update.Example
This trigger demonstrates
Comments within the code explain the functionality of the trigger.
CREATE TRIGGER audit_employee AFTER INSERT OR DELETE OR UPDATE ON emp FOR EACH ROW BEGIN /* AUDITPACKAGE is a package with a public package variable REASON. REASON could be set by the application by a command such as EXECUTE AUDITPACKAGE.SET_REASON(reason_string). Note that a package variable has state for the duration of a session and that each session has a separate copy of all package variables. */ IF auditpackage.reason IS NULL THEN raise_application_error(-20201, 'Must specify reason' || ' with AUDITPACKAGE.SET_REASON(reason_string)'); END IF; /* If the above conditional evaluates to TRUE, the user-specified error number and message is raised, the trigger stops execution, and the effects of the triggering statement are rolled back. Otherwise, a new row is inserted into the predefined auditing table named AUDIT_EMPLOYEE containing the existing and new values of the EMP table and the reason code defined by the REASON variable of AUDITPACKAGE. Note that the "old" values are NULL if triggering statement is an INSERT and the "new" values are NULL if the triggering statement is a DELETE. */ INSERT INTO audit_employee VALUES (:old.ssn, :old.name, :old.job_classification, :old.sal, :new.ssn, :new.name, :new.job_classification, :new.sal, auditpackage.reason, user, sysdate ); END;
Optionally, you can also set the reason code back to
NULL if you wanted to force the reason code to be set for every update. The following simple AFTER statement trigger sets the reason code back to NULL after the triggering statement is executed:CREATE TRIGGER audit_employee_reset AFTER INSERT OR DELETE OR UPDATE ON emp BEGIN auditpackage.set_reason(NULL); END;
Notice that the previous two triggers are both fired by the same type of SQL statement. However, the
AFTER row trigger is fired once for each row of the table affected by the triggering statement, while the AFTER statement trigger is fired only once after the triggering statement execution is completed.
Another example of using triggers to do auditing is shown below. This trigger tracks changes being made to the
EMP table, and stores this information in AUDIT_TABLE and AUDIT_TABLE_VALUES.CREATE OR REPLACE TRIGGER audit_emp AFTER INSERT OR UPDATE OR DELETE ON emp FOR EACH ROW DECLARE time_now DATE; terminal CHAR(10); BEGIN -- get current time, and the terminal of the user time_now := SYSDATE; terminal := USERENV('TERMINAL'); -- record new employee primary key IF INSERTING THEN INSERT INTO audit_table VALUES (audit_seq.NEXTVAL, user, time_now, terminal, 'EMP', 'INSERT', :new.empno); -- record primary key of the deleted row ELSIF DELETING THEN INSERT INTO audit_table VALUES (audit_seq.NEXTVAL, user, time_now, terminal, 'EMP', 'DELETE', :old.empno); -- for updates, record the primary key -- of the row being updated ELSE INSERT INTO audit_table VALUES (audit_seq.NEXTVAL, user, time_now, terminal, 'EMP', 'UPDATE', :old.empno); -- and for SAL and DEPTNO, record old and new values IF UPDATING ('SAL') THEN INSERT INTO audit_table_values VALUES (audit_seq.CURRVAL, 'SAL', :old.sal, :new.sal); ELSIF UPDATING ('DEPTNO') THEN INSERT INTO audit_table_values VALUES (audit_seq.CURRVAL, 'DEPTNO', :old.deptno, :new.deptno); END IF; END IF; END; /
Integrity Constraints and Triggers
Triggers and declarative integrity constraints can both be used to constrain data input. However, triggers and integrity constraints have significant differences.
Declarative integrity constraints are statements about the database that are always true. A constraint applies to existing data in the table and any statement that manipulates the table.
Triggers constrain what a transaction can do. A trigger does not apply to data loaded before the definition of the trigger; therefore, it is not known if all data in a table conforms to the rules established by an associated trigger.
Although triggers can be written to enforce many of the same rules supported by Oracle's declarative integrity constraint features, triggers should only be used to enforce complex business rules that cannot be defined using standard integrity constraints. The declarative integrity constraint features provided with Oracle offer the following advantages when compared to constraints defined by triggers:
While most aspects of data integrity can be defined and enforced using declarative integrity constraints, triggers can be used to enforce complex business constraints not definable using declarative integrity constraints. For example, triggers can be used to enforce
UPDATEandDELETESETNULL, andUPDATEandDELETESETDEFAULTreferential actions- referential integrity when the parent and child tables are on different nodes of a distributed database
- complex check constraints not definable using the expressions allowed in a
CHECKconstraint
Enforcing Referential Integrity Using Triggers
Many cases of referential integrity can be enforced using triggers. However, only use triggers when you want to enforce the
UPDATE and DELETE SET NULL (when referenced data is updated or deleted, all associated dependent data is site to NULL), and UPDATE and DELETE SET DEFAULT (when referenced data is updated or deleted, all associated dependent data is set to a default value) referential actions, or when you want to enforce referential integrity between parent and child tables on different nodes of a distributed database.
When using triggers to maintain referential integrity, declare the
PRIMARY (or UNIQUE) KEY constraint in the parent table. If referential integrity is being maintained between a parent and child table in the same database, you can also declare the foreign key in the child table, but disable it; this prevents the corresponding PRIMARY KEY constraint from being dropped (unless the PRIMARY KEYconstraint is explicitly dropped with the CASCADE option).
To maintain referential integrity using triggers:
- A trigger must be defined for the child table that guarantees values inserted or updated in the foreign key correspond to values in the parent key.
- One or more triggers must be defined for the parent table. These triggers guarantee the desired referential action (
RESTRICT,CASCADE, orSETNULL) for values in the foreign key when values are updated or deleted in the parent key. No action is required for inserts into the parent table (no dependent foreign keys exist).
The following sections provide examples of the triggers necessary to enforce referential integrity. The
EMP and DEPT table relationship is used in these examples.
Several of the triggers include statements that lock rows (
SELECT... FOR UPDATE). This operation is necessary to maintain concurrency as the rows are being processed.Foreign Key Trigger for Child Table
The following trigger guarantees that before an
INSERT or UPDATE statement affects a foreign key value, the corresponding value exists in the parent key. The mutating table exception included in the example below allows this trigger to be used with the UPDATE_SET_DEFAULT and UPDATE_CASCADE triggers. This exception can be removed if this trigger is used alone.CREATE TRIGGER emp_dept_check BEFORE INSERT OR UPDATE OF deptno ON emp FOR EACH ROW WHEN (new.deptno IS NOT NULL) -- Before a row is inserted, or DEPTNO is updated in the EMP -- table, fire this trigger to verify that the new foreign -- key value (DEPTNO) is present in the DEPT table. DECLARE dummy INTEGER; -- used for cursor fetch below invalid_department EXCEPTION; valid_department EXCEPTION; mutating_table EXCEPTION; PRAGMA EXCEPTION_INIT (mutating_table, -4091); -- Cursor used to verify parent key value exists. If -- present, lock parent key's row so it can't be -- deleted by another transaction until this -- transaction is committed or rolled back. CURSOR PRINT_SALARY_CHANGES_cursor (dn NUMBER) IS SELECT deptno FROM dept WHERE deptno = dn FOR UPDATE OF deptno; BEGIN OPEN dummy_cursor (:new.deptno); FETCH dummy_cursor INTO dummy; -- Verify parent key. If not found, raise user-specified -- error number and message. If found, close cursor -- before allowing triggering statement to complete. IF dummy_cursor%NOTFOUND THEN RAISE invalid_department; ELSE RAISE valid_department; END IF; CLOSE dummy_cursor; EXCEPTION WHEN invalid_department THEN CLOSE dummy_cursor; raise_application_error(-20000, 'Invalid Department' || ' Number' || TO_CHAR(:new.deptno)); WHEN valid_department THEN CLOSE dummy_cursor; WHEN mutating_table THEN NULL; END;
UPDATE and DELETE RESTRICT Trigger for the Parent Table
The following trigger is defined on the
DEPT table to enforce the UPDATE and DELETE RESTRICT referential action on the primary key of the DEPT table:CREATE TRIGGER dept_restrict BEFORE DELETE OR UPDATE OF deptno ON dept FOR EACH ROW -- Before a row is deleted from DEPT or the primary key -- (DEPTNO) of DEPT is updated, check for dependent -- foreign key values in EMP; rollback if any are found. DECLARE dummy INTEGER; -- used for cursor fetch below employees_present EXCEPTION; employees_not_present EXCEPTION; -- Cursor used to check for dependent foreign key values. CURSOR dummy_cursor (dn NUMBER) IS SELECT deptno FROM emp WHERE deptno = dn; BEGIN OPEN dummy_cursor (:old.deptno); FETCH dummy_cursor INTO dummy; -- If dependent foreign key is found, raise user-specified -- error number and message. If not found, close cursor -- before allowing triggering statement to complete. IF dummy_cursor%FOUND THEN RAISE employees_present; /* dependent rows exist */ ELSE RAISE employees_not_present; /* no dependent rows */ END IF; CLOSE dummy_cursor; EXCEPTION WHEN employees_present THEN CLOSE dummy_cursor; raise_application_error(-20001, 'Employees Present in' || ' Department ' || TO_CHAR(:old.deptno)); WHEN employees_not_present THEN CLOSE dummy_cursor; END;
UPDATE and DELETE SET NULL Triggers for Parent Table
The following trigger is defined on the
DEPT table to enforce the UPDATE and DELETE SET NULL referential action on the primary key of the DEPT table:CREATE TRIGGER dept_set_null AFTER DELETE OR UPDATE OF deptno ON dept FOR EACH ROW -- Before a row is deleted from DEPT or the primary key -- (DEPTNO) of DEPT is updated, set all corresponding -- dependent foreign key values in EMP to NULL. BEGIN IF UPDATING AND :OLD.deptno != :NEW.deptno OR DELETING THEN UPDATE emp SET emp.deptno = NULL WHERE emp.deptno = :old.deptno; END IF; END;
DELETE Cascade Trigger for Parent Table
The following trigger on the
DEPT table enforces the DELETE CASCADE referential action on the primary key of the DEPT table:CREATE TRIGGER dept_del_cascade AFTER DELETE ON dept FOR EACH ROW -- Before a row is deleted from DEPT, delete all -- rows from the EMP table whose DEPTNO is the same as -- the DEPTNO being deleted from the DEPT table. BEGIN DELETE FROM emp WHERE emp.deptno = :old.deptno; END;
Note:
Typically, the code for DELETE cascade is combined with the code for UPDATE SET NULL or UPDATE SET DEFAULT to account for both updates and deletes.
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UPDATE Cascade Trigger for Parent Table
The following trigger ensures that if a department number is updated in the
DEPT table, this change is propagated to dependent foreign keys in the EMP table:-- Generate a sequence number to be used as a flag for -- determining if an update has occurred on a column. CREATE SEQUENCE update_sequence INCREMENT BY 1 MAXVALUE 5000 CYCLE; CREATE PACKAGE integritypackage AS updateseq NUMBER; END integritypackage; CREATE or replace PACKAGE BODY integritypackage AS END integritypackage; ALTER TABLE emp ADD update_id NUMBER; -- create flag col. CREATE TRIGGER dept_cascade1 BEFORE UPDATE OF deptno ON dept DECLARE dummy NUMBER; -- Before updating the DEPT table (this is a statement -- trigger), generate a new sequence number and assign -- it to the public variable UPDATESEQ of a user-defined -- package named INTEGRITYPACKAGE. BEGIN SELECT update_sequence.NEXTVAL INTO dummy FROM dual; integritypackage.updateseq := dummy; END; CREATE TRIGGER dept_cascade2 AFTER DELETE OR UPDATE OF deptno ON dept FOR EACH ROW -- For each department number in DEPT that is updated, -- cascade the update to dependent foreign keys in the -- EMP table. Only cascade the update if the child row -- has not already been updated by this trigger. BEGIN IF UPDATING THEN UPDATE emp SET deptno = :new.deptno, update_id = integritypackage.updateseq /*from 1st*/ WHERE emp.deptno = :old.deptno AND update_id IS NULL; /* only NULL if not updated by the 3rd trigger fired by this same triggering statement */ END IF; IF DELETING THEN -- Before a row is deleted from DEPT, delete all -- rows from the EMP table whose DEPTNO is the same as -- the DEPTNO being deleted from the DEPT table. DELETE FROM emp WHERE emp.deptno = :old.deptno; END IF; END; CREATE TRIGGER dept_cascade3 AFTER UPDATE OF deptno ON dept BEGIN UPDATE emp SET update_id = NULL WHERE update_id = integritypackage.updateseq; END;
Enforcing Complex Check Constraints
Triggers can enforce integrity rules other than referential integrity. For example, this trigger performs a complex check before allowing the triggering statement to execute. Comments within the code explain the functionality of the trigger.
CREATE TRIGGER salary_check BEFORE INSERT OR UPDATE OF sal, job ON emp FOR EACH ROW DECLARE minsal NUMBER; maxsal NUMBER; salary_out_of_range EXCEPTION; BEGIN /* Retrieve the minimum and maximum salary for the employee's new job classification from the SALGRADE table into MINSAL and MAXSAL. */ SELECT minsal, maxsal INTO minsal, maxsal FROM salgrade WHERE job_classification = :new.job; /* If the employee's new salary is less than or greater than the job classification's limits, the exception is raised. The exception message is returned and the pending INSERT or UPDATE statement that fired the trigger is rolled back. */ IF (:new.sal < minsal OR :new.sal > maxsal) THEN RAISE salary_out_of_range; END IF; EXCEPTION WHEN salary_out_of_range THEN raise_application_error (-20300, 'Salary '||TO_CHAR(:new.sal)||' out of range for ' ||'job classification '||:new.job ||' for employee '||:new.name); WHEN NO_DATA_FOUND THEN raise_application_error(-20322, 'Invalid Job Classification ' ||:new.job_classification); END;
Complex Security Authorizations and Triggers
Triggers are commonly used to enforce complex security authorizations for table data. Only use triggers to enforce complex security authorizations that cannot be defined using the database security features provided with Oracle. For, example, a trigger can prohibit updates to salary data of the
EMP table during weekends, holidays, and non-working hours.
When using a trigger to enforce a complex security authorization, it is best to use a
BEFORE statement trigger. Using a BEFORE statement trigger has these benefits:- The security check is done before the triggering statement is allowed to execute so that no wasted work is done by an unauthorized statement.
- The security check is performed only once for the triggering statement, not for each row affected by the triggering statement.
Example
This example shows a trigger used to enforce security. The Comments within the code explain the functionality of the trigger.
CREATE TRIGGER emp_permit_changes BEFORE INSERT OR DELETE OR UPDATE ON emp DECLARE dummy INTEGER; not_on_weekends EXCEPTION; not_on_holidays EXCEPTION; non_working_hours EXCEPTION; BEGIN /* check for weekends */ IF (TO_CHAR(sysdate, 'DY') = 'SAT' OR TO_CHAR(sysdate, 'DY') = 'SUN') THEN RAISE not_on_weekends; END IF; /* check for company holidays */ SELECT COUNT(*) INTO dummy FROM company_holidays WHERE TRUNC(day) = TRUNC(sysdate); /* TRUNC gets rid of time parts of dates */ IF dummy > 0 THEN RAISE not_on_holidays; END IF; /* Check for work hours (8am to 6pm) */ IF (TO_CHAR(sysdate, 'HH24') < 8 OR TO_CHAR(sysdate, 'HH24') > 18) THEN RAISE non_working_hours; END IF; EXCEPTION WHEN not_on_weekends THEN raise_application_error(-20324,'May not change ' ||'employee table during the weekend'); WHEN not_on_holidays THEN raise_application_error(-20325,'May not change ' ||'employee table during a holiday'); WHEN non_working_hours THEN raise_application_error(-20326,'May not change ' ||'emp table during non-working hours'); END;
Transparent Event Logging and Triggers
Triggers are very useful when you want to transparently perform a related change in the database following certain events.
Example
The
REORDER trigger example on page 13-21 shows a trigger that reorders parts as necessary when certain conditions are met (that is, a triggering statement is issued and the PARTS_ON_HAND value is less than the REORDER_POINT value).Derived Column Values and Triggers
Triggers can derive column values automatically based upon a value provided by an
INSERT or UPDATE statement. This type of trigger is useful to force values in specific columns that depend on the values of other columns in the same row. BEFORE row triggers are necessary to complete this type of operation because- the dependent values must be derived before the insert or update occurs so that the triggering statement can use the derived values.
- the trigger must fire for each row affected by the triggering
INSERTorUPDATEstatement.
Example
The following example illustrates how a trigger can be used to derive new column values for a table whenever a row is inserted or updated. Comments within the code explain its functionality.
BEFORE INSERT OR UPDATE OF ename ON emp /* Before updating the ENAME field, derive the values for the UPPERNAME and SOUNDEXNAME fields. Users should be restricted from updating these fields directly. */ FOR EACH ROW BEGIN :new.uppername := UPPER(:new.ename); :new.soundexname := SOUNDEX(:new.ename); END;
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