For Example: The price of a product changes constantly. It is important to maintain the history of the prices of the products.
We can create a trigger to update the 'product_price_history' table when the price of the product is updated in the 'product' table.
1) Create the 'product' table and 'product_price_history' table
CREATE TABLE product_price_history
(product_id number(5),
product_name varchar2(32),
supplier_name varchar2(32),
unit_price number(7,2) );
CREATE TABLE product
(product_id number(5),
product_name varchar2(32),
supplier_name varchar2(32),
unit_price number(7,2) );
2) Create the price_history_trigger and execute it.
CREATE or REPLACE TRIGGER price_history_trigger
BEFORE UPDATE OF unit_price
ON product
FOR EACH ROW
BEGIN
INSERT INTO product_price_history
VALUES
(:old.product_id,
:old.product_name,
:old.supplier_name,
:old.unit_price);
END;
/
3) Lets update the price of a product.
UPDATE PRODUCT SET unit_price = 800 WHERE product_id = 100
Once the above update query is executed, the trigger fires and updates the 'product_price_history' table.
4)If you ROLLBACK the transaction before committing to the database, the data inserted to the table is also rolled back.
Types of PL/SQL Triggers
There are two types of triggers based on the which level it is triggered.
1) Row level trigger - An event is triggered for each row upated, inserted or deleted.
2) Statement level trigger - An event is triggered for each sql statement executed.
PL/SQL Trigger Execution Hierarchy
The following hierarchy is followed when a trigger is fired.
1) BEFORE statement trigger fires first.
2) Next BEFORE row level trigger fires, once for each row affected.
3) Then AFTER row level trigger fires once for each affected row. This events will alternates between BEFORE and AFTER row level triggers.
4) Finally the AFTER statement level trigger fires.
For Example: Let's create a table 'product_check' which we can use to store messages when triggers are fired.
CREATE TABLE product
(Message varchar2(50),
Current_Date number(32)
);
Let's create a BEFORE and AFTER statement and row level triggers for the product table.
1) BEFORE UPDATE, Statement Level: This trigger will insert a record into the table 'product_check' before a sql update statement is executed, at the statement level.
CREATE or REPLACE TRIGGER Before_Update_Stat_product
BEFORE
UPDATE ON product
Begin
INSERT INTO product_check
Values('Before update, statement level',sysdate);
END;
/
2) BEFORE UPDATE, Row Level: This trigger will insert a record into the table 'product_check' before each row is updated.
CREATE or REPLACE TRIGGER Before_Upddate_Row_product
BEFORE
UPDATE ON product
FOR EACH ROW
BEGIN
INSERT INTO product_check
Values('Before update row level',sysdate);
END;
/
3) AFTER UPDATE, Statement Level: This trigger will insert a record into the table 'product_check' after a sql update statement is executed, at the statement level.
CREATE or REPLACE TRIGGER After_Update_Stat_product
AFTER
UPDATE ON product
BEGIN
INSERT INTO product_check
Values('After update, statement level', sysdate);
End;
/
4) AFTER UPDATE, Row Level: This trigger will insert a record into the table 'product_check' after each row is updated.
CREATE or REPLACE TRIGGER After_Update_Row_product
AFTER
insert On product
FOR EACH ROW
BEGIN
INSERT INTO product_check
Values('After update, Row level',sysdate);
END;
/
Now lets execute a update statement on table product.
UPDATE PRODUCT SET unit_price = 800
WHERE product_id in (100,101);
Lets check the data in 'product_check' table to see the order in which the trigger is fired.
SELECT * FROM product_check;
Output:
Mesage Current_Date
------------------------------------------------------------
Before update, statement level 26-Nov-2008
Before update, row level 26-Nov-2008
After update, Row level 26-Nov-2008
Before update, row level 26-Nov-2008
After update, Row level 26-Nov-2008
After update, statement level 26-Nov-2008
The above result shows 'before update' and 'after update' row level events have occured twice, since two records were updated. But 'before update' and 'after update' statement level events are fired only once per sql statement.
The above rules apply similarly for INSERT and DELETE statements.
How To know Information about Triggers.
We can use the data dictionary view 'USER_TRIGGERS' to obtain information about any trigger.
The below statement shows the structure of the view 'USER_TRIGGERS'
DESC USER_TRIGGERS;
NAME Type
--------------------------------------------------------
TRIGGER_NAME VARCHAR2(30)
TRIGGER_TYPE VARCHAR2(16)
TRIGGER_EVENT VARCHAR2(75)
TABLE_OWNER VARCHAR2(30)
BASE_OBJECT_TYPE VARCHAR2(16)
TABLE_NAME VARCHAR2(30)
COLUMN_NAME VARCHAR2(4000)
REFERENCING_NAMES VARCHAR2(128)
WHEN_CLAUSE VARCHAR2(4000)
STATUS VARCHAR2(8)
DESCRIPTION VARCHAR2(4000)
ACTION_TYPE VARCHAR2(11)
TRIGGER_BODY LONG
This view stores information about header and body of the trigger.
SELECT * FROM user_triggers WHERE trigger_name = 'Before_Update_Stat_product';
The above sql query provides the header and body of the trigger 'Before_Update_Stat_product'.
You can drop a trigger using the following command.
DROP TRIGGER trigger_name;
CYCLIC CASCADING in a TRIGGER
This is an undesirable situation where more than one trigger enter into an infinite loop. while creating a trigger we should ensure the such a situtation does not exist.
The below example shows how Trigger's can enter into cyclic cascading.
Let's consider we have two tables 'abc' and 'xyz'. Two triggers are created.
1) The INSERT Trigger, triggerA on table 'abc' issues an UPDATE on table 'xyz'.
2) The UPDATE Trigger, triggerB on table 'xyz' issues an INSERT on table 'abc'.
In such a situation, when there is a row inserted in table 'abc', triggerA fires and will update table 'xyz'.
When the table 'xyz' is updated, triggerB fires and will insert a row in table 'abc'.
This cyclic situation continues and will enter into a infinite loop, which will crash the database.
References:
http://plsql-tutorial.com/plsql-triggers.htm
https://docs.oracle.com/cd/B19306_01/appdev.102/b14251/adfns_triggers.htm
