A BEFORE UPDATE Trigger means that Oracle will fire this trigger before the UPDATE operation is executed.
The syntax for an BEFORE UPDATE Trigger is:
CREATE or REPLACE TRIGGER trigger_name
BEFORE UPDATE
ON table_name
[ FOR EACH ROW ]
DECLARE
-- variable declarations
BEGIN
-- trigger code
EXCEPTION
WHEN ...
-- exception handling
END;
trigger_name is the name of the trigger to create.
Restrictions:
You can not create a BEFORE trigger on a view.
You can update the :NEW values.
You can not update the :OLD values.
For example:
If you had a table created as follows:
CREATE TABLE orders
( order_id number(5),
quantity number(4),
cost_per_item number(6,2),
total_cost number(8,2),
updated_date date,
updated_by varchar2(10)
);
We could then create a BEFORE UPDATE trigger as follows:
CREATE OR REPLACE TRIGGER orders_before_update
BEFORE UPDATE
ON orders
FOR EACH ROW
DECLARE
v_username varchar2(10);
BEGIN
-- Find username of person performing UPDATE on the table
SELECT user INTO v_username
FROM dual;
-- Update updated_date field to current system date
:new.updated_date := sysdate;
-- Update updated_by field to the username of the person performing the UPDATE
:new.updated_by := v_username;
END;
From www.techonthenet.com