This article shows you how to use BEFORE UPDATE TRIGGER, it’s fire before the update operation is executed. In real life scenarios, it is mostly used for purposes like:

1. Data validation
2. Update values automatically
3. Data logging, or auditing

1. Data Validation

Suppose some companies have job openings and already having application data and the criteria are :

1. Job Experience must be more than or equal to 3 years and
2. Previous application attempt must not be done in last 2 years.

To ensure data integrity, We will create BEFORE UPDATE trigger and it will restrict UPDATE on data which violates any of the above criteria.

1.1 First, we will create table job_openings.

--Creating job_openings table.
CREATE TABLE job_openings
    APPLICATION_ID number(10) primary key,
    FIRST_NAME varchar2(50),
    LAST_NAME varchar2(50),
    JOB_EXPERIENCE number(2),
    LAST_APPLIED_DATE date
);

1.2 Then we will create a before update tigger on column JOB_EXPERIENCE and LAST_APPLIED_DATE of job_openings table.

-- Creating TRIGGER
CREATE OR REPLACE TRIGGER trg_before_emp_update
BEFORE UPDATE OF JOB_EXPERIENCE,LAST_APPLIED_DATE
  on job_openings
  FOR EACH ROW 
DECLARE
years_since_last_applied number(5);
BEGIN
years_since_last_applied := -1;
  IF(:NEW.LAST_APPLIED_DATE IS NOT NULL) THEN
    SELECT MONTHS_BETWEEN(TO_DATE(sysdate,'DD-MON-YYYY'), TO_DATE(:NEW.LAST_APPLIED_DATE,'DD-MON-YYYY'))/12
      INTO years_since_last_applied FROM dual;
   -- Check whether years_since_last_applied is greater than 2 years or not
    IF (years_since_last_applied <= 2) THEN
      RAISE_APPLICATION_ERROR(-20000,'Previous application attempt must not be done in last 2 years.');
    END IF;
  END IF;
    -- Job experience must be more than or equal to 3 years.
    IF(:new.JOB_EXPERIENCE < 3) THEN
      RAISE_APPLICATION_ERROR(-20000,'Job experience must be more than or equal to 3 years.');
    END IF;          
END;

1.3 Normal Data.

-- setting date format to to 'DD-MON-YYYY'
alter session set nls_date_format = 'DD-MON-YYYY';
INSERT INTO job_openings VALUES (1,'Mark','Sharma',10,'01-JAN-2012');
INSERT INTO job_openings VALUES (2,'Praveen','Kumar',4,'01-DEC-2010');
INSERT INTO job_openings VALUES (3,'Rahul','Kohli',6,null);
-- output
1 rows inserted.
1 rows inserted.
1 rows inserted.

select * from job_openings;

1.4 Test Trigger raise error – Job experience must be more than or equal to 3 years.

-- Try to update job_experience less than 3 years
UPDATE job_openings SET JOB_EXPERIENCE = 2 where APPLICATION_ID = 1;
-- error 
Error report -
ORA-20000: Job experience must be more than or equal to 3 years.
ORA-06512: at "SYSTEM.TRG_BEFORE_EMP_UPDATE", line 21
ORA-04088: error during execution of trigger 'SYSTEM.TRG_BEFORE_EMP_UPDATE'

1.5 Test Trigger raise error – Previous application attempt must not be done in last 2 years.

-- Try to update last_Applied_Date which is less than 2 years
UPDATE job_openings SET LAST_APPLIED_DATE = '10-JUN-2016' where APPLICATION_ID = 3;
-- error
Error report -
ORA-20000: Previous application attempt must not be done in last 2 years.
ORA-06512: at "SYSTEM.TRG_BEFORE_EMP_UPDATE", line 15
ORA-04088: error during execution of trigger 'SYSTEM.TRG_BEFORE_EMP_UPDATE'

2. Update values

A BEFORE UPDATE trigger example to update :NEW.UPDATED_BY and :NEW.UPDATED_DATE automatically.

2.1 Create a table.

--Creating person_records table.
CREATE TABLE person_records
    PERSON_ID number(10) primary key,
    FIRST_NAME varchar2(50),
    LAST_NAME varchar2(50),
    HIRE_DATE date,
    UPDATED_BY varchar2(20),
    UPDATED_DATE date
);

2.2 Create a before update trigger on table person_records

CREATE OR REPLACE TRIGGER trg_before_person_update
BEFORE UPDATE
  on person_records
  FOR EACH ROW 
DECLARE
username varchar2(20);
BEGIN
  SELECT USER INTO username FROM dual;
  -- Setting updated_by and updated_Date values.
  :NEW.UPDATED_BY := username;
  :NEW.UPDATED_DATE := sysdate;
END;

2.3 Insert two records.

-- setting date format to to 'DD-MON-YYYY'
alter session set nls_date_format = 'DD-MON-YYYY';
INSERT INTO person_records VALUES (101,'Devil','Khedut',sysdate,null,null);
INSERT INTO person_records VALUES (102,'Kanji','Yadav',sysdate,null,null);
-- output
1 rows inserted.
1 rows inserted.

select * from person_records;

2.4 Update records and it will fires the before update trigger. The values of UPDATED_BY and UPDATED_DATE will be updated automatically.

UPDATE person_records SET first_name = 'Lavji' WHERE person_id = 101;
UPDATE person_records SET first_name = 'Jogi' WHERE person_id = 102;
-- output
1 rows updated.
1 rows updated.

select * from person_records;

3. Logging/Auditing data

In this example, we have created a trigger which will insert rows into an audit table before each updating of transaction table. Whenever user UPDATE data of bank_transactions, the old data will be inserted into bank_transactions_audit by trigger for audit or backup purpose.

3.1 Create a bank transaction table.

--Creating bank_transactions table.
CREATE TABLE bank_transactions
    TXN_ID number(10) primary key,
    TXN_NUMBER varchar2(20),
    PARTY_NAME varchar2(50),
    TXN_DATE date,
    CREATED_BY varchar2(20),
    CREATED_DATE date
);

3.2 Create another bank transaction audit table.

--Creating bank_transactions_audit table.
CREATE TABLE bank_transactions_audit
    TXN_ID number(10),
    TXN_NUMBER varchar2(20),
    PARTY_NAME varchar2(50),
    TXN_DATE date,
    CREATED_BY varchar2(20),
    CREATED_DATE date
);

3.3 Create a before update trigger on bank_transactions table.

--Creating Trigger
CREATE OR REPLACE TRIGGER trg_before_update_txn_audit
BEFORE UPDATE
  ON bank_transactions
  FOR EACH ROW
BEGIN
  -- Insert OLD values in audit table for logging purpose
  INSERT INTO bank_transactions_audit VALUES(:OLD.TXN_ID,:OLD.TXN_NUMBER,
    :OLD.PARTY_NAME,:OLD.TXN_DATE,:OLD.CREATED_BY,:OLD.CREATED_DATE);
END;

3.4 Insert some records.

--Inserting values
INSERT INTO BANK_TRANSACTIONS values ('1','TXN1234','Peter Thomas','12-MAY-2017','HR',sysdate);
INSERT INTO BANK_TRANSACTIONS values ('2','TXN9999','Jemes Patel','10-JUN-2016','HR',sysdate);
select * from BANK_TRANSACTIONS;

3.5 Insert operation didn't fire the before update trigger.

select * from BANK_TRANSACTIONS_AUDIT;
-- output
no rows selected.

3.6 Update records, it will fires the "before update" trigger and insert the old data into another audit table.

--updating values.
UPDATE bank_transactions SET txn_number = 'NEWTXN8080' WHERE txn_id = 1;
UPDATE bank_transactions SET txn_number = 'NEWTXN9595' WHERE txn_id = 2;
-- output
1 rows updated.
1 rows updated.

select * from BANK_TRANSACTIONS;

select * from BANK_TRANSACTIONS_AUDIT;

References

1. Oracle - CREATE TRIGGER Statement
2. Oracle - PL/SQL Triggers