8 Using Triggers

About OLD and NEW Pseudorecords

When a row-level trigger fires, the PL/SQL run-time 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 8-1.

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.

This trigger is part of the sample application that the tutorials and examples in this document show how to develop and deploy.

To create EVALUATIONS_LOG and EVAL_CHANGE_TRIGGER:

1. Create the EVALUATIONS_LOG table:

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

2. 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;
   

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

The sequence EVALUATIONS_SEQ, created in "Creating and Managing Sequences", generates primary keys for the EVALUATIONS table. 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_seq. 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.

This trigger is part of the sample application that the tutorials and examples in this document show how to develop and deploy.

To create the NEW_EVALUATION trigger:

1. On the Connections tab, expand hr_conn.

   Under the hr_conn icon, a list of schema object types appears.

2. Right-click Triggers.

   A list of choices appears.

3. Click New Trigger.

   The Create Trigger window opens. The field Schema has the value HR and the field Name has the default value TRIGGER1.

4. In the Name field, type NEW_EVALUATION_TRIGGER over the default value.

5. Click the tab Trigger.

   The Trigger pane appears. By default, the field Trigger Type has the value TABLE, the check box Enabled is selected, the field Table Owner has the value HR, the field Table Name has the value COUNTRIES, the options Before and Statement Level are selected, the options After and Row Level are deselected, and the check boxes Insert, Update, and Delete are deselected.

6. In the field Table Name, from the drop-down menu, select EVALUATIONS.

7. Select the option Row Level.

   The option Statement Level is now deselected.

8. Select the check box Insert.

9. 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;
   

10. In the CREATE TRIGGER statement, replace NULL with this:

    :NEW.evaluation_id := evaluations_seq.NEXTVAL
    

    The title of the NEW_EVALUATION_TRIGGER pane is in italic font, indicating that the trigger is not yet saved in the database.

11. 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.

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, 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 EMP_LOCATIONS.NAME, you must update EMPLOYEES.LAST_NAME and EMPLOYEES.FIRST_NAME. This is what the INSTEAD OF trigger in Example 8-1 does.

This trigger is part of the sample application that the tutorials and examples in this document show how to develop and deploy.

NEW and OLD are pseudorecords that the PL/SQL run-time 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.

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;

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 a BEFORE trigger, and hr_logoff_trigger is an AFTER trigger.

These triggers are not part of the sample application that the tutorials and examples in this document show how to develop and deploy.

To create HR_USERS_LOG, HR_LOGON_TRIGGER, and HR_LOGOFF_TRIGGER:

1. Create the HR_USERS_LOG table:

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

2. 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;
   

3. 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;