PL/SQL Triggers

PL/SQL stands for Procedural Language/ Structured Query Language. It has block structure programming features.PL/SQL supports SQL queries. It also supports the declaration of the variables, control statements, Functions, Records, Cursor, Procedure, and Triggers.PL/SQL contains a declaration section, execution section, and exception-handling section. Declare and exception handling sections are optional.

Syntax:

Declaration section

BEGIN

Execution section

EXCEPTION

Exception section

END;

PL/SQL Triggers

PL/SQL triggers are block structures and predefined programs invoked automatically when some event occurs. They are stored in the database and invoked repeatedly in a particular scenario. There are two states of the triggers, they are enabled and disabled. When the trigger is created it is enabled. CREATE TRIGGER statement creates a trigger. A triggering event is specified on a table, a view, a schema, or a database.BEFORE and AFTER are the trigger Timing points.DML triggers are created on a table or view, and triggers. Crossedition triggers are created on Edition-based redefinition. System Triggers are created on schema or database using DDL or database operation statements.It is applied on new data only ,it don't affect existing data.

They are associated with response-based events such as a

• Database Definition Language statements such as CREATE, DROP or ALTER.
  
• Database Manipulation Language statements such as UPDATE, INSERT or DELETE.
  
• Database operations such as LOGON, LOGOFF, STARTUP, and SHUTDOWN .

Why are Triggers important?

The importance of Triggers are:

• Automated Action: It helps to automate actions in response to events on table or views.
  
• Data integrity: Constraint can be applied to the data with the help of trigger.It is used to ensure referential integrity.
  
  
• Consistency: It helps to maintain the consistency of the database by performing immediate responses to specific events.
  
• Error handling: It helps in error handling by responding to the errors. For example, If specific condition is not met it will provide an error message.

PL/SQL Trigger Structure

Triggers are fired on the tables or views which are in the database. Either table, view ,schema, or a database are the basic requirement to execute a trigger. The trigger is specified first and then the action statement are specified later.

Syntax:

CREATE OR REPLACE TRIGGER trigger_name

BEFORE or AFTER or INSTEAD OF //trigger timings

INSERT or UPDATE or DELETE // Operation to be performed

of column_name

on Table_name

FOR EACH ROW

DECLARE

Declaration section

BEGIN

Execution section

EXCEPTION

Exception section

END;

/

Query operation to be performed i.e INSERT,DELETE,UPDATE.

• CREATE [ OR REPLACE ] TRIGGER trigger_name is used to create a trigger or replace the existing trigger.|
  
• BEFORE | AFTER | INSTEAD OF specifies trigger timing.
  
• INSERT | UPDATE | DELETE are the DML operations performed on table or views.
  
• OF column_name specifies the column that would be updated.
  
• ON table_name species the table for the operation.
  
• FOR EACH ROW specify that trigger is executed on each row .

Types of PL/SQL Triggers

Trigger timing and operations forms different combinations such as BEFORE INSERT OR BEFORE DELETE OR BEFORE UPDATE .BEFORE and AFTER are known as conditional triggers.

Conditional Trigger: Before

Trigger is activated before the operation on the table or view is performed.

Query:

-- Create Geeks table
CREATE TABLE Geeks (
    Id INT,
    Name VARCHAR2(20),
    Score INT
);

-- Insert into Geeks Table 
INSERT INTO Geeks (Id, Name, Score) VALUES (1, 'Sam', 800);
INSERT INTO Geeks (Id, Name, Score) VALUES (2, 'Ram', 699);
INSERT INTO Geeks (Id, Name, Score) VALUES (3, 'Tom', 250);
INSERT INTO Geeks (Id, Name, Score) VALUES (4, 'Om', 350);
INSERT INTO Geeks (Id, Name, Score) VALUES (5, 'Jay', 750);
-- insert statement should be written for each entry in Oracle Sql Developer

CREATE TABLE Affect (
    Id INT,
    Name VARCHAR2(20),
    Score INT
);

-- BEFORE INSERT trigger
CREATE OR REPLACE TRIGGER BEFORE_INSERT
BEFORE INSERT ON Geeks
FOR EACH ROW
BEGIN
    INSERT INTO Affect (Id, Name, Score)
    VALUES (:NEW.Id, :NEW.Name, :NEW.Score);
END;
/
INSERT INTO Geeks (Id, Name, Score) VALUES (6, 'Arjun', 500);

BEFORE DELETE Trigger

-- BEFORE DELETE trigger
CREATE OR REPLACE TRIGGER BEFORE_DELETE
BEFORE DELETE ON Geeks
FOR EACH ROW
BEGIN
    INSERT INTO Affect (Id, Name, Score)
    VALUES (:OLD.Id, :OLD.Name, :OLD.Score);
END;
/
DELETE FROM Geeks WHERE Id = 3;

BEFORE UPDATE Trigger

-- BEFORE UPDATE trigger
CREATE OR REPLACE TRIGGER BEFORE_UPDATE
BEFORE UPDATE ON Geeks
FOR EACH ROW
BEGIN
    INSERT INTO Affect (Id, Name, Score)
    VALUES (:OLD.Id, :OLD.Name, :OLD.Score);
END;
/
UPDATE Geeks SET Score = 900 WHERE Id = 5;
SELECT * FROM Affect; 
SELECT * FROM Geeks; 

Output:

Explanation:

• BEFORE_INSERT Trigger is fired before adding a row in Geeks Table,and row is inserted in the Affect table.
  
• BEFORE_DELETE Trigger is activated before the row is delete from the Geeks table and row which satisfy the condition is added to Affect table.
  
• BEFORE_UPDATE TRIGGER is activated before the row with Id=5 is updated and row with old values is added to Affect table

Geeks table after trigger events

Conditional Trigger: After

Trigger is activated after the operation on the table or view is performed.

Query:

SET SERVEROUTPUT ON;


CREATE TABLE Geeks (
    Id INT,
    Name VARCHAR2(20),
    Score INT
);


-- Insert into Geeks Table 
INSERT INTO Geeks (Id, Name, Score) VALUES (1, 'Sam', 800);
INSERT INTO Geeks (Id, Name, Score) VALUES (2, 'Ram', 699);
INSERT INTO Geeks (Id, Name, Score) VALUES (3, 'Tom', 250);
INSERT INTO Geeks (Id, Name, Score) VALUES (4, 'Om', 350);
INSERT INTO Geeks (Id, Name, Score) VALUES (5, 'Jay', 750);
-- insert statement should be written for each entry in Oracle Sql Developer


CREATE TABLE Affect (
    Id INT,
    Name VARCHAR2(20),
    Score INT
);
SELECT * FROM Geeks; 
-- AFTER DELETE trigger
CREATE OR REPLACE TRIGGER AFTER_DELETE
AFTER DELETE ON Geeks
FOR EACH ROW
BEGIN
    INSERT INTO Affect (Id, Name, Score)
    VALUES (:OLD.Id, :OLD.Name, :OLD.Score);
END;
/
DELETE FROM Geeks WHERE Id = 4;


-- AFTER UPDATE trigger
CREATE OR REPLACE TRIGGER AFTER_UPDATE
AFTER UPDATE ON Geeks
FOR EACH ROW
BEGIN
    INSERT INTO Affect (Id, Name, Score)
    VALUES (:NEW.Id, :NEW.Name, :NEW.Score);
END;
/
UPDATE Geeks SET Score = 1050 WHERE Id = 5;
SELECT * FROM Affect; 
SELECT * FROM Geeks; 

Output:

Explanation: After the deletion of the row from the Geek table trigger is fired and the row which is deleted is added to the Affect Table.In second trigger i.e After_update trigger is fired after performing update on Geeks table and the row is added to Affect Table.Output contains the Affect table and the Geek table after the trigger events.

Common Use Cases of PL/SQL Triggers

• To automate the actions in response to the events and reducing manual task.
  
• To apply constraint to ensure referential integrity and to prevent invalid data in table or database.
  
• In error handling to response to errors.

Conclusion

In conclusion, Oracle PL/SQL triggers offer a powerful way to automate actions before or after specified events, such as INSERT, UPDATE or DELETE, on a table. By executing custom code, handling exceptions, and enforcing business rules, triggers improve database performance and data integrity.