What is a database Trigger?

A trigger is a pl/sql block structure which is fired when a DML statements like Insert, Delete, Update is executed on a database table. A trigger is triggered automatically when an associated DML statement is executed. Syntax of Triggers Syntax for Creating a Trigger CREATE [OR REPLACE ] TRIGGER trigger_name {BEFORE | AFTER | INSTEAD OF } {INSERT [OR] | UPDATE [OR] | DELETE} [OF col_name] ON table_name [REFERENCING OLD AS o NEW AS n] [FOR EACH ROW] WHEN (condition) BEGIN --- sql statements END;

• CREATE [OR REPLACE ] TRIGGER trigger_name - This clause creates a trigger with the given name or overwrites an existing trigger with the same name.
• {BEFORE | AFTER | INSTEAD OF } - This clause indicates at what time should the trigger get fired. i.e for example: before or after updating a table. INSTEAD OF is used to create a trigger on a view. before and after cannot be used to create a trigger on a view.
• {INSERT [OR] | UPDATE [OR] | DELETE} - This clause determines the triggering event. More than one triggering events can be used together separated by OR keyword. The trigger gets fired at all the specified triggering event.
• [OF col_name] - This clause is used with update triggers. This clause is used when you want to trigger an event only when a specific column is updated.
• CREATE [OR REPLACE ] TRIGGER trigger_name - This clause creates a trigger with the given name or overwrites an existing trigger with the same name.
• [ON table_name] - This clause identifies the name of the table or view to which the trigger is associated.
• [REFERENCING OLD AS o NEW AS n] - This clause is used to reference the old and new values of the data being changed. By default, you reference the values as :old.column_name or :new.column_name. The reference names can also be changed from old (or new) to any other user-defined name. You cannot reference old values when inserting a record, or new values when deleting a record, because they do not exist.
• [FOR EACH ROW] - This clause is used to determine whether a trigger must fire when each row gets affected ( i.e. a Row Level Trigger) or just once when the entire sql statement is executed(i.e.statement level Trigger).
• WHEN (condition) - This clause is valid only for row level triggers. The trigger is fired only for rows that satisfy the condition specified.

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