Triggers in DBMS in Hindi + English

Triggers in DBMS

Trigger ek automatic SQL command hota hai jo tab run hota hai jab koi specific event (INSERT, UPDATE, DELETE) kisi table par perform kiya jata hai.

Trigger tab chalta hai jab:

Table me naya record insert hota hai
Table ka data update hota hai
Table se data delete hota hai

Triggers kyu use karte hain?

Triggers ka use hum:

-> Rules ya logic automatically apply karne ke liye karte hain
-> Data ko safe aur consistent rakhne ke liye karte hain
-> Changes ka record rakhne ke liye karte hain (logging)
-> Dusre tables ko auto update karne ke liye karte hain

Trigger kaise kaam karta hai?

Types of Triggers:

1. BEFORE Trigger

Ye trigger tab chalta hai jab actual operation se pehle kuch karna ho.
Jaise ki data check karna ya validate karna.

2. AFTER Trigger

Ye trigger tab chalta hai jab operation ho chuka ho.
Jaise ki changes ko kisi log table me record karna.

3. INSTEAD OF Trigger

Ye mainly views me use hota hai.
Ye normal operation ki jagah kuch aur kaam karta hai.

SQL Example: Salary Update ka Trigger

Maan lo humare paas 2 tables hain:

Table 1: Employees

CREATE TABLE Employees (
    EmpID INT PRIMARY KEY,
    Name VARCHAR(50),
    Salary INT
);

Table 2: Audit_Log

CREATE TABLE Audit_Log (
    LogID INT PRIMARY KEY AUTO_INCREMENT,
    EmpID INT,
    OldSalary INT,
    NewSalary INT,
    ModifiedDate DATETIME
);

Trigger banate hain jo salary update hone par Audit_Log me entry kare:

CREATE TRIGGER AfterSalaryUpdate
AFTER UPDATE ON Employees
FOR EACH ROW
BEGIN
   INSERT INTO Audit_Log(EmpID, OldSalary, NewSalary, ModifiedDate)
   VALUES (OLD.EmpID, OLD.Salary, NEW.Salary, NOW());
END;

Iska matlab:

Jab bhi kisi employee ki salary update hogi:

OLD salary record ki jayegi
NEW salary record ki jayegi
Audit_Log table me yeh sab auto store ho jayega

Triggers ke Fayde (Advantages)

Automation: Repeated kaam auto ho jata hai
Consistency: Har baar same rule follow hota hai
Security: Galat data se bachav
Logging: Changes ka record automatically banta hai

Triggers ke Nuksaan (Disadvantages)

Hidden logic: Kabhi-kabhi samajhna mushkil hota hai ki kya kaam trigger se ho raha hai
Debugging tough: Problem find karna thoda tricky hota hai
Performance: Agar zyada triggers ho gaye to database slow ho sakta hai

Summary Table:

Feature	Explanation
Trigger kya hai?	Auto SQL code jo event pe chalta hai
Trigger kab chalta hai?	INSERT, UPDATE, DELETE ke time
Types of Triggers	BEFORE, AFTER, INSTEAD OF
Use kisliye hota hai?	Logging, Validation, Auto Updates

The Hingineer