Oracle / PLSQL: BEFORE DELETE Trigger

This Oracle tutorial explains how to create a BEFORE DELETE Trigger in Oracle with syntax and examples.

Description

A BEFORE DELETE Trigger means that Oracle will fire this trigger before the DELETE operation is executed.

Syntax

The syntax to create a BEFORE DELETE Trigger in Oracle/PLSQL is:

CREATE [ OR REPLACE ] TRIGGER trigger_name
BEFORE DELETE
   ON table_name
   [ FOR EACH ROW ]

DECLARE
   -- variable declarations

BEGIN
   -- trigger code

EXCEPTION
   WHEN ...
   -- exception handling

END;

Parameters or Arguments

OR REPLACE

Optional. If specified, it allows you to re-create the trigger is it already exists so that you can change the trigger definition without issuing a DROP TRIGGER statement.

trigger_name

The name of the trigger to create.

BEFORE DELETE

It indicates that the trigger will fire before the DELETE operation is executed.

table_name

The name of the table that the trigger is created on.

Restrictions

• You can not create a BEFORE trigger on a view.
• You can update the :NEW values.
• You can not update the :OLD values.

Note

• See also how to create AFTER DELETE, AFTER INSERT, AFTER UPDATE, BEFORE INSERT, and BEFORE UPDATE triggers.
• See also how to drop a trigger.

Example

Let's look at an example of how to create an BEFORE DELETE trigger using the CREATE TRIGGER statement.

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)
);

We could then use the CREATE TRIGGER statement to create an BEFORE DELETE trigger as follows:

TIP: When using SQLPlus, you need to enter slash on a new line after the trigger. Otherwise, the script won't execute.

CREATE OR REPLACE TRIGGER orders_before_delete
BEFORE DELETE
   ON orders
   FOR EACH ROW

DECLARE
   v_username varchar2(10);

BEGIN

   -- Find username of person performing the DELETE on the table
   SELECT user INTO v_username
   FROM dual;

   -- Insert record into audit table
   INSERT INTO orders_audit
   ( order_id,
     quantity,
     cost_per_item,
     total_cost,
     delete_date,
     deleted_by )
   VALUES
   ( :old.order_id,
     :old.quantity,
     :old.cost_per_item,
     :old.total_cost,
      sysdate,
      v_username );

END;

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