1️⃣3️⃣ 🧠 MySQL Triggers & Stored Programs – Automate & Encapsulate Logic

MySQL allows you to encapsulate logic within the database using Triggers, Stored Procedures, and Stored Functions, minimizing repetitive code, enhancing consistency, and improving data integrity.

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🧲 Introduction – Why Use Triggers and Stored Programs?

Triggers and stored programs allow developers to move business logic into the database, automating actions like logging, validation, or batch processing. This leads to:

🔄 Reduced application complexity
🧩 Better data integrity enforcement
🚀 Improved performance and maintainability

🎯 In this guide, you’ll learn:

• What triggers, procedures, and functions are
• How to write and manage them in MySQL
• Real-world usage scenarios
• Best practices for optimal implementation

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📘 Topics Covered

🔹 Topic	📄 Description
🔔 Triggers	Run SQL automatically on INSERT, UPDATE, DELETE
🧮 Stored Procedures	Reusable logic with input/output parameters
🔁 Stored Functions	Return values for use in queries
🧰 Manage Stored Programs	View, drop, and maintain routines
⚖️ Triggers vs Procedures	Side-by-side comparison
🚀 Use Cases	Practical business applications
📘 Best Practices	Tips to avoid pitfalls
📌 Summary & FAQ	Recap and common questions

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🔔 1. MySQL Triggers – Respond to Table Events

📌 What is a Trigger?

A trigger runs SQL code automatically in response to data changes.

CREATE TRIGGER trigger_name
{BEFORE | AFTER} {INSERT | UPDATE | DELETE}
ON table_name FOR EACH ROW
BEGIN
  -- SQL logic
END;

✅ Example – Audit Log

CREATE TRIGGER after_user_update
AFTER UPDATE ON users
FOR EACH ROW
BEGIN
  INSERT INTO user_audit(user_id, old_email, new_email, updated_at)
  VALUES (OLD.id, OLD.email, NEW.email, NOW());
END;

💡 Used for logging, validation, or historical tracking.

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🧮 2. MySQL Stored Procedures – Encapsulate SQL Logic

📌 What is a Stored Procedure?

A stored procedure is a named set of SQL statements stored in the DB and executed via CALL.

DELIMITER //

CREATE PROCEDURE add_customer(
  IN name VARCHAR(100),
  IN email VARCHAR(100)
)
BEGIN
  INSERT INTO customers(name, email) VALUES (name, email);
END //

DELIMITER ;

▶️ Call Procedure

CALL add_customer('Alice', 'alice@example.com');

✅ Great for form actions, business rules, and batch jobs.

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🔁 3. MySQL Stored Functions – Return a Value

📌 What is a Stored Function?

A stored function returns a result and can be used in expressions.

DELIMITER //

CREATE FUNCTION get_discount(price DECIMAL(10,2))
RETURNS DECIMAL(10,2)
DETERMINISTIC
BEGIN
  RETURN price * 0.90;
END //

DELIMITER ;

✅ Use in Query

SELECT product_name, get_discount(price) AS discounted_price
FROM products;

🎯 Use functions inside SELECT, WHERE, or ORDER BY clauses.

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🧰 4. Manage & View Stored Programs

🔍 Command	💡 Purpose
SHOW TRIGGERS;	Lists all triggers
SHOW PROCEDURE STATUS;	Lists procedures
SHOW FUNCTION STATUS;	Lists functions
DROP PROCEDURE/FUNCTION/TRIGGER;	Deletes specific object

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⚖️ Triggers vs Stored Programs

Feature	Triggers	Procedures/Functions
Executes Automatically	✅ Yes	❌ No (manual)
Takes Input Parameters	❌ No	✅ Yes
Returns Value	❌ No	✅ (Functions only)
Ideal Use	Logging, Auditing	Business logic, calculations

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🚀 Real-World Use Cases

Scenario	Tool	Benefit
Audit changes	Trigger	No extra app logic
Insert data + validation	Procedure	Consistent rules
Calculate total w/ discount	Function	Use in SELECT queries
Archive deletes	AFTER DELETE Trigger	Soft delete strategy

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📘 Best Practices

✅ Tip	💡 Why
Keep triggers lightweight	Avoid slowdowns
Use DETERMINISTIC for functions	Helps optimizer
Document params & logic	Improves maintainability
Avoid heavy logic in triggers	Use procedures instead
Use DELIMITER // for routines	Prevent semicolon conflicts

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📌 Summary – Recap & Next Steps

Triggers, procedures, and functions empower MySQL to automate and encapsulate logic, improve code reuse, and support scalable architecture across your applications.

🔍 Key Takeaways

• Triggers run automatically on table events
• Stored procedures encapsulate logic with parameters
• Functions return values and are query-friendly
• Use these tools to move business logic closer to the data

⚙️ Real-World Relevance
Found in audit systems, ERP workflows, e-commerce logic, data validations, and performance-heavy operations where central DB logic is preferred.

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❓ FAQ – MySQL Triggers & Stored Programs

❓ Can a stored procedure return a value?

✅ Not directly, but you can use OUT parameters or SELECT statements inside.

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❓ Can I create multiple triggers on one table?

✅ Yes, but only one trigger per timing/event type. (e.g., one BEFORE INSERT, one AFTER INSERT)

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❓ Can I use stored functions inside WHERE clauses?

✅ Absolutely. They behave like built-in SQL functions:

SELECT * FROM orders WHERE get_discount(total) > 100;

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❓ Can a trigger update the same table?

⚠️ Yes, but use caution. MySQL disables recursive triggers by default.

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❓ Are stored routines faster than app logic?

✅ For repeated DB logic, yes—they reduce round trips and centralize business rules.

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