Oracle PL/SQL – INSTEAD OF Trigger example

This article shows you how to use INSTEAD OF TRIGGER to update

1. A noneditioning view
2. A nested table column of a noneditioning view

This INSTEAD OF triggers provide a way to modify views that cannot be modified directly through DML statements like INSERT, UPDATE and DELETE

An INSTEAD OF trigger is

1. Always a row-level trigger.
2. Can read OLD and NEW values, but cannot change them.
3. Cannot be conditional. Means we can not add WHEN or IF condition.

1. Insert Data into a View

In this example, we have created a INSTEAD OF trigger which will insert rows into respective tables of a view when we execute the insert statement on a view.

1.1 Create tables.

CREATE TABLE customer_details
	customer_id number(10) primary key,
	customer_name varchar2(20),
	country varchar2(20)
);

CREATE TABLE projects_details
	project_id number(10) primary key,
	project_name varchar2(30),
	project_start_Date date,
	customer_id number(10) references customer_details(customer_id)
);

1.2 Create customer_projects_view view to get results of customers and their projects.

CREATE OR REPLACE VIEW customer_projects_view AS
   SELECT cust.customer_id, cust.customer_name, cust.country,
          projectdtls.project_id, projectdtls.project_name, 
		  projectdtls.project_start_Date
   FROM customer_details cust, projects_details projectdtls
   WHERE cust.customer_id = projectdtls.customer_id;

1.3 If we INSERT INTO customer_projects_view view directly, the database will raise the following error :

INSERT INTO customer_projects_view VALUES (1,'XYZ Enterprise','Japan',101,'Library management',sysdate);
-- output
SQL Error: ORA-01779: cannot modify a column which maps to a non key-preserved table
01779. 00000 -  "cannot modify a column which maps to a non key-preserved table"
*Cause:    An attempt was made to insert or update columns of a join view which
           map to a non-key-preserved table.
*Action:   Modify the underlying base tables directly.

1.4 Instead, we should create a INSTEAD OF trigger on the view to perform the actual operation.

CREATE OR REPLACE TRIGGER trg_cust_proj_view_insert
   INSTEAD OF INSERT ON customer_projects_view
   DECLARE
     duplicate_info EXCEPTION;
     PRAGMA EXCEPTION_INIT (duplicate_info, -00001);
   BEGIN
   INSERT INTO customer_details
       (customer_id,customer_name,country)
     VALUES (:new.customer_id, :new.customer_name, :new.country);
   INSERT INTO projects_details (project_id, project_name, project_start_Date, customer_id)
   VALUES (
     :new.project_id,
     :new.project_name,
     :new.project_start_Date,
     :new.customer_id);
   EXCEPTION
     WHEN duplicate_info THEN
       RAISE_APPLICATION_ERROR (
         num=> -20107,
         msg=> 'Duplicate customer or project id');
   END trg_cust_proj_view_insert;

1.5 Insert into view again. The INSTEAD OF trigger will be fired and insert the data into the actual table.

INSERT INTO customer_projects_view VALUES (1,'XYZ Enterprise','Japan',101,'Library management',sysdate);
INSERT INTO customer_projects_view VALUES (2,'ABC Infotech','India',202,'HR management',sysdate);

1.6 Select table.

SELECT * FROM customer_details;

SELECT * FROM projects_details;

1.7 Select view.

SELECT * FROM customer_projects_view;

2. Nested table view column

Example to insert data into a nested table view column with INSTEAD OF Trigger.

2.1 Create table, type, nested table view column for testing.

CREATE TABLE vehicle_mfg_company_details (
  company_id number(10) primary key,
  company_name varchar2(50) NOT NULL
);

CREATE TABLE vehicle_details (
  vehicle_id  number(10) primary key,
  company_id  number(10) references vehicle_mfg_company_details(company_id),
  vehicle_model_name varchar2(50) NOT NULL
);

CREATE OR REPLACE TYPE nestedTableEle
IS
OBJECT (
  vehicle_id NUMBER(10),
  vehicle_model_name VARCHAR2(50)
  );

-- nested table view column
CREATE OR REPLACE TYPE vehicle_details_list_ IS
  TABLE OF nestedTableEle;

2.2 Create a View.

CREATE OR REPLACE VIEW company_vehicles_view AS
  SELECT company.company_id, 
         company.company_name,
         CAST (MULTISET (SELECT vehicle.vehicle_id, vehicle.vehicle_model_name
                         FROM vehicle_details vehicle
                         WHERE vehicle.company_id = company.company_id
                        AS vehicle_details_list_
              ) vehiclelist
FROM vehicle_mfg_company_details company;

2.3 Insert data into nested table view column.

-- no error.
INSERT INTO vehicle_mfg_company_details VALUES (101,'Ford');
-- error
INSERT INTO TABLE (
  SELECT vw.vehiclelist 
  FROM company_vehicles_view vw
  WHERE company_id = 101
VALUES (1, 'EcoSport');
-- output
SQL Error: ORA-25015: cannot perform DML on this nested table view column
25015. 00000 -  "cannot perform DML on this nested table view column"
*Cause:    DML cannot be performed on a nested table view column except through
           an INSTEAD OF trigger
*Action:   Create an INSTEAD OF trigger over the nested table view column
           and then perform the DML.

2.4 Create a INSTEAD OF trigger

CREATE OR REPLACE TRIGGER trg_comp_vehicles_view_insrt
  INSTEAD OF INSERT ON NESTED TABLE vehiclelist OF company_vehicles_view
  REFERENCING NEW AS Vehicle
              PARENT AS Company
  FOR EACH ROW
BEGIN
  -- Insert on nested table translates to insert on base table.
  INSERT INTO vehicle_details (
    vehicle_id,
    company_id,
    vehicle_model_name
  VALUES (
    :Vehicle.vehicle_id,
    :Company.company_id,
    :Vehicle.vehicle_model_name
  );
END;

2.5 Insert data into nested table view column again.

INSERT INTO TABLE (
  SELECT vw.vehiclelist 
  FROM company_vehicles_view vw
  WHERE company_id = 101
VALUES (1, 'EcoSport');
INSERT INTO TABLE (
  SELECT vw.vehiclelist 
  FROM company_vehicles_view vw
  WHERE company_id = 101
VALUES (2, 'Endeavour');

2.6 Display table.

select * from vehicle_mfg_company_details;

select * from vehicle_details;

References

1. Instead OF Trigger:- Oracle official docs
2. PL/SQL Triggers :- Oracle official docs
3. Oracle PL/SQL – Before UPDATE Trigger example