Using Triggers

6.1 About Triggers

A trigger is a PL/SQL unit that is stored in the database and (if it is in the enabled state) automatically executes ("fires") in response to a specified event.

A trigger has this structure:

TRIGGER trigger_name
  triggering_event
  [ trigger_restriction ]
BEGIN
  triggered_action;
END;

The trigger_name must be unique for triggers in the schema. A trigger can have the same name as another kind of object in the schema (for example, a table); however, Oracle recommends using a naming convention that avoids confusion.

If the trigger is in the enabled state, the triggering_event causes the database to execute the triggered_action if the trigger_restriction is either TRUE or omitted. The triggering_event is associated with either a table, a view, a schema, or the database, and it is one of these:

DML statement (described in "About Data Manipulation Language (DML) Statements")
DDL statement (described in "About Data Definition Language (DDL) Statements")
Database operation (SERVERERROR, LOGON, LOGOFF, STARTUP, or SHUTDOWN)

If the trigger is in the disabled state, the triggering_event does not cause the database to execute the triggered_action, even if the trigger_restriction is TRUE or omitted.

By default, a trigger is created in the enabled state. You can disable an enabled trigger, and enable a disabled trigger.

Unlike a subprogram, a trigger cannot be invoked directly. A trigger is invoked only by its triggering event, which can be caused by any user or application. You might be unaware that a trigger is executing unless it causes an error that is not handled properly.

A simple trigger can fire at exactly one of these timing points:

Before the triggering event executes (statement-level BEFORE trigger)
After the triggering event executes (statement-level AFTER trigger)
Before each row that the event affects (row-level BEFORE trigger)
After each row that the event affects (row-level AFTER trigger)

A compound trigger can fire at multiple timing points. For information about compound triggers, see Oracle Database PL/SQL Language Reference.

An INSTEAD OF trigger is defined on a view, and its triggering event is a DML statement. Instead of executing the DML statement, Oracle Database executes the INSTEAD OF trigger. For more information, see "Creating an INSTEAD OF Trigger".

A system trigger is defined on a schema or the database. A trigger defined on a schema fires for each event associated with the owner of the schema (the current user). A trigger defined on a database fires for each event associated with all users.

One use of triggers is to enforce business rules that apply to all client applications. For example, suppose that data added to the EMPLOYEES table must have a certain format, and that many client applications can add data to this table. A trigger on the table can ensure the proper format of all data added to it. Because the trigger executes whenever any client adds data to the table, no client can circumvent the rules, and the code that enforces the rules can be stored and maintained only in the trigger, rather than in every client application. For other uses of triggers, see Oracle Database PL/SQL Language Reference.

See Also:

Oracle Database PL/SQL Language Reference for complete information about triggers

6.2 Creating Triggers

To create triggers, use either the SQL Developer graphical interface or the DDL statement CREATE TRIGGER.

This section shows how to use both of these ways to create triggers.

By default, a trigger is created in the enabled state. To create a trigger in disabled state, use the CREATE TRIGGER statement with the DISABLE clause.

Note:

To create triggers, you must have appropriate privileges; however, for this discussion, you do not need this additional information.

Note:

To do the tutorials in this document, you must be connected to Oracle Database as the user HR from SQL Developer.

See Also:

Oracle Database PL/SQL Language Reference for more information about the CREATE TRIGGER statement
"Editing Installation Scripts that Create Triggers"

6.2.1 About OLD and NEW Pseudorecords

When a row-level trigger fires, the PL/SQL runtime system creates and populates the two pseudorecords OLD and NEW. They are called pseudorecords because they have some, but not all, of the properties of records.

For the row that the trigger is processing:

For an INSERT trigger, OLD contains no values, and NEW contains the new values.
For an UPDATE trigger, OLD contains the old values, and NEW contains the new values.
For a DELETE trigger, OLD contains the old values, and NEW contains no values.

To reference a pseudorecord, put a colon before its name—:OLD or :NEW—as in Example 6-1.

See Also:

Oracle Database PL/SQL Language Reference for more information about OLD and NEW pseudorecords

6.2.2 Tutorial: Creating a Trigger that Logs Table Changes

This tutorial shows how to use the CREATE TRIGGER statement to create a trigger, EVAL_CHANGE_TRIGGER, which adds a row to the table EVALUATIONS_LOG whenever an INSERT, UPDATE, or DELETE statement changes the EVALUATIONS table.

The trigger adds the row after the triggering statement executes, and uses the conditional predicates INSERTING, UPDATING, and DELETING to determine which of the three possible DML statements fired the trigger.

EVAL_CHANGE_TRIGGER is a statement-level trigger and an AFTER trigger.

To create EVALUATIONS_LOG and EVAL_CHANGE_TRIGGER:

Create the EVALUATIONS_LOG table:

CREATE TABLE EVALUATIONS_LOG ( log_date DATE
                             , action VARCHAR2(50));

Create EVAL_CHANGE_TRIGGER:

CREATE OR REPLACE TRIGGER EVAL_CHANGE_TRIGGER
  AFTER INSERT OR UPDATE OR DELETE
  ON EVALUATIONS
DECLARE
  log_action  EVALUATIONS_LOG.action%TYPE;
BEGIN
  IF INSERTING THEN
    log_action := 'Insert';
  ELSIF UPDATING THEN
    log_action := 'Update';
  ELSIF DELETING THEN
    log_action := 'Delete';
  ELSE
    DBMS_OUTPUT.PUT_LINE('This code is not reachable.');
  END IF;

  INSERT INTO EVALUATIONS_LOG (log_date, action)
    VALUES (SYSDATE, log_action);
END;

See Also:

Oracle Database PL/SQL Language Reference for more information about conditional predicates

6.2.3 Tutorial: Creating a Trigger that Generates a Primary Key for a Row Before It Is Inserted

This tutorial shows how to use the SQL Developer Create Trigger tool to create a trigger that fires before a row is inserted into the EVALUATIONS table, and generates the unique number for the primary key of that row, using EVALUATIONS_SEQUENCE.

The sequence EVALUATIONS_SEQUENCE (created in "Tutorial: Creating a Sequence") generates primary keys for the EVALUATIONS table (created in "Creating Tables"). However, these primary keys are not inserted into the table automatically.

This tutorial shows how to use the SQL Developer Create Trigger tool to create a trigger named NEW_EVALUATION_TRIGGER, which fires before a row is inserted into the EVALUATIONS table, and generates the unique number for the primary key of that row, using EVALUATIONS_SEQUENCE. The trigger fires once for each row affected by the triggering INSERT statement.

NEW_EVALUATION_TRIGGER is a row-level trigger and a BEFORE trigger.

To create the NEW_EVALUATION trigger:

In the Connections frame, expand hr_conn.
In the list of schema object types, right-click Triggers.
In the list of choices, click New Trigger.
In the Create Trigger window:
1. In the Name field, type NEW_EVALUATION_TRIGGER over the default value TRIGGER1.
2. For Base Object, select EVALUATIONS from the menu.
3. Move INSERT from Available Events to Selected Events.
  
  (Select INSERT and click >.)
4. Deselect the option Statement Level.
5. Click OK.
  
  The NEW_EVALUATION_TRIGGER pane opens, showing the CREATE TRIGGER statement that created the trigger:
```
CREATE OR REPLACE
TRIGGER NEW_EVALUATION_TRIGGER
BEFORE INSERT ON EVALUATIONS
FOR EACH ROW
BEGIN
  NULL;
END;
```
  The title of the NEW_EVALUATION_TRIGGER pane is in italic font, indicating that the trigger is not yet saved in the database.

In the CREATE TRIGGER statement, replace NULL with this:

:NEW.evaluation_id := evaluations_sequence.NEXTVAL

From the File menu, select Save.

Oracle Database compiles the procedure and saves it. The title of the NEW_EVALUATION_TRIGGER pane is no longer in italic font.

6.2.4 Creating an INSTEAD OF Trigger

A view presents the output of a query as a table. If you want to change a view as you would change a table, then you must create INSTEAD OF triggers. Instead of changing the view, they change the underlying tables.

For example, consider the view EMP_LOCATIONS, whose NAME column is created from the LAST_NAME and FIRST_NAME columns of the EMPLOYEES table:

CREATE VIEW EMP_LOCATIONS AS
SELECT e.EMPLOYEE_ID,
  e.LAST_NAME || ', ' || e.FIRST_NAME NAME,
  d.DEPARTMENT_NAME DEPARTMENT,
  l.CITY CITY,
  c.COUNTRY_NAME COUNTRY
FROM EMPLOYEES e, DEPARTMENTS d, LOCATIONS l, COUNTRIES c
WHERE e.DEPARTMENT_ID = d.DEPARTMENT_ID AND
 d.LOCATION_ID = l.LOCATION_ID AND
 l.COUNTRY_ID = c.COUNTRY_ID
ORDER BY LAST_NAME;

To update the view EMP_LOCATIONS.NAME (created in "Creating Views with the CREATE VIEW Statement"), you must update EMPLOYEES.LAST_NAME and EMPLOYEES.FIRST_NAME. This is what the INSTEAD OF trigger in Example 6-1 does.

NEW and OLD are pseudorecords that the PL/SQL runtime engine creates and populates whenever a row-level trigger fires. OLD and NEW store the original and new values, respectively, of the record being processed by the trigger. They are called pseudorecords because they do not have all properties of PL/SQL records.

Example 6-1 Creating an INSTEAD OF Trigger

CREATE OR REPLACE TRIGGER update_name_view_trigger
INSTEAD OF UPDATE ON emp_locations
BEGIN
  UPDATE employees SET
    first_name = substr( :NEW.name, instr( :new.name, ',' )+2),
    last_name = substr( :NEW.name, 1, instr( :new.name, ',')-1)
  WHERE employee_id = :OLD.employee_id;
END;

See Also:

Oracle Database PL/SQL Language Reference for more information about INSTEAD OF triggers
Oracle Database PL/SQL Language Reference for more information about OLD and NEW

6.2.5 Tutorial: Creating Triggers that Log LOGON and LOGOFF Events

This tutorial shows how to use the CREATE TRIGGER statement to create two triggers, HR_LOGON_TRIGGER and HR_LOGOFF_TRIGGER. After someone logs on as user HR, HR_LOGON_TRIGGER adds a row to the table HR_USERS_LOG. Before someone logs off as user HR, HR_LOGOFF_TRIGGER adds a row to the table HR_USERS_LOG.

HR_LOGON_TRIGGER and HR_LOGOFF_TRIGGER are system triggers. HR_LOGON_TRIGGER is an AFTER trigger and HR_LOGOFF_TRIGGER is a BEFORE trigger.

To create HR_USERS_LOG, HR_LOGON_TRIGGER, and HR_LOGOFF_TRIGGER:

Create the HR_USERS_LOG table:

CREATE TABLE hr_users_log (
  user_name VARCHAR2(30),
  activity VARCHAR2(20),
  event_date DATE
);

Create HR_LOGON_TRIGGER:

CREATE OR REPLACE TRIGGER hr_logon_trigger
  AFTER LOGON
  ON HR.SCHEMA
BEGIN
  INSERT INTO hr_users_log (user_name, activity, event_date)
  VALUES (USER, 'LOGON', SYSDATE);
END;

Create HR_LOGOFF_TRIGGER:

CREATE OR REPLACE TRIGGER hr_logoff_trigger
  BEFORE LOGOFF
  ON HR.SCHEMA
BEGIN
  INSERT INTO hr_users_log (user_name, activity, event_date)
  VALUES (USER, 'LOGOFF', SYSDATE);
END;

See Also:

Oracle Database PL/SQL Language Reference for more information about system triggers

6.3 Changing Triggers

To change a trigger, use either the SQL Developer tool Edit or the DDL statement CREATE TRIGGER with the OR REPLACE clause.

To change a trigger using the Edit tool:

In the Connections frame, expand hr_conn.
In the list of schema object types, expand Triggers.
In the list of triggers, click the trigger to change.
In the frame to the right of the Connections frame, the Code pane appears, showing the code that created the trigger.

The Code pane is in write mode. (Clicking the pencil icon switches the mode from write mode to read only, or the reverse.)
In the Code pane, change the code.

The title of the pane is in italic font, indicating that the change is not yet saved in the database.
From the File menu, select Save.

Oracle Database compiles the trigger and saves it. The title of the pane is no longer in italic font.

See Also:

"About Data Definition Language (DDL) Statements" for general information that applies to the CREATE OR REPLACE TRIGGER statement
Oracle Database PL/SQL Language Reference for more information about the CREATE OR REPLACE TRIGGER statement

6.4 Disabling and Enabling Triggers

You might need to temporarily disable triggers that reference objects that are unavailable, or to upload a large amount of data without the delay that triggers cause (as in a recovery operation). After the referenced objects become available, or you have finished uploading the data, you can re-enable the triggers.

See Also:

Oracle Database PL/SQL Language Reference for more information about the ALTER TRIGGER statement
Oracle Database SQL Language Reference for more information about the ALTER TABLE statement

6.4.1 Disabling or Enabling a Single Trigger

To disable or enable a single trigger, use either the Disable Trigger or Enable Trigger tool or the ALTER TRIGGER statement with the DISABLE or ENABLE clause.

For example, these statements disable and enable the eval_change_trigger:

ALTER TRIGGER eval_change_trigger DISABLE;
ALTER TRIGGER eval_change_trigger ENABLE;

To use the Disable Trigger or Enable Trigger tool:

In the Connections frame, expand hr_conn.
In the list of schema object types, expand Triggers.
In the list of triggers, right-click the desired trigger.
In the list of choices, select Disable or Enable.
In the Disable or Enable window, click Apply.
In the Confirmation window, click OK.

6.4.2 Disabling or Enabling All Triggers on a Single Table

To disable or enable all triggers on a specific table, use either the Disable All Triggers or Enable All Triggers tool or the ALTER TABLE statement with the DISABLE ALL TRIGGERS or ENABLE ALL TRIGGERS clause.

For example, these statements disable and enable all triggers on the evaluations table:

ALTER TABLE evaluations DISABLE ALL TRIGGERS;
ALTER TABLE evaluations ENABLE ALL TRIGGERS;

To use the Disable All Triggers or Enable All Triggers tool:

In the Connections frame, expand hr_conn.
In the list of schema object types, expand Tables.
In the list of tables, right-click the desired table.
In the list of choices, select Triggers.
In the list of choices, select Disable All or Enable All.
In the Disable All or Enable All window, click Apply.
In the Confirmation window, click OK.

6.5 About Trigger Compilation and Dependencies

Compiled triggers depend on the schema objects on which they are defined. If an object on which a trigger depends is dropped, or changed such that there is a mismatch between the trigger and the object, then the trigger is invalidated.

Running a CREATE TRIGGER statement compiles the trigger being created. If this compilation causes an error, then the CREATE TRIGGER statement fails. To see the compilation errors, use this statement:

SELECT * FROM USER_ERRORS WHERE TYPE = 'TRIGGER';

Compiled triggers depend on the schema objects on which they are defined. For example, NEW_EVALUATION_TRIGGER depends on the EVALUATIONS table:

CREATE OR REPLACE
TRIGGER NEW_EVALUATION_TRIGGER
BEFORE INSERT ON EVALUATIONS
FOR EACH ROW
BEGIN
  :NEW.evaluation_id := evaluations_seq.NEXTVAL;
END;

To see the schema objects on which triggers depend, use this statement:

SELECT * FROM ALL_DEPENDENCIES WHERE TYPE = 'TRIGGER';

If an object on which a trigger depends is dropped, or changed such that there is a mismatch between the trigger and the object, then the trigger is invalidated. The next time the trigger is invoked, it is recompiled. To recompile a trigger immediately, use the ALTER TRIGGER statement with the COMPILE clause. For example:

ALTER TRIGGER NEW_EVALUATION_TRIGGER COMPILE;

See Also:

Oracle Database PL/SQL Language Reference for more information about trigger compilation and dependencies

6.6 Dropping Triggers

You must drop a trigger before dropping the objects on which it depends.

To drop a trigger, use either the SQL Developer Connections frame and Drop tool, or the DDL statement DROP TRIGGER.

This statement drops the trigger EVAL_CHANGE_TRIGGER:

DROP TRIGGER EVAL_CHANGE_TRIGGER;

To drop a trigger using the Drop tool:

In the Connections frame, expand hr_conn.
In the list of schema object types, expand Triggers.
In the list of triggers, right-click the name of the trigger to drop.
In the list of choices, click Drop Trigger.
In the Drop window, click Apply.
In the Confirmation window, click y.

See Also:

"About Data Definition Language (DDL) Statements" for general information that applies to the DROP TRIGGER statement
Oracle Database PL/SQL Language Reference for information about the DROP TRIGGER statement