🚨 MySQL SIGNAL & RESIGNAL – Raise Custom Errors in Stored Programs

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🧲 Introduction – Why Use SIGNAL & RESIGNAL?

In MySQL, SIGNAL and RESIGNAL are used to raise custom error messages and manage exception handling inside stored procedures, functions, and triggers. These statements help:

• 🚫 Stop execution when invalid conditions are met
• 🔍 Report meaningful error messages to the application
• 🛡️ Enforce business rules at the database level
• 🔁 Re-throw caught errors for debugging or logging

🎯 In this guide, you’ll learn:

• What SIGNAL and RESIGNAL do
• Syntax for raising custom SQL errors
• How to handle and rethrow exceptions
• Best practices and real-world use cases

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⚠️ 1. What Is SIGNAL?

The SIGNAL statement raises a user-defined exception. It stops execution and returns a specific SQLSTATE, error message, or condition.

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🔹 Basic Syntax

SIGNAL SQLSTATE '45000'
SET MESSAGE_TEXT = 'Custom error message';

• '45000': Generic “unhandled user-defined exception” code
• MESSAGE_TEXT: The error message shown to the client or app

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🔹 Example – Reject Invalid Age

IF age < 18 THEN
  SIGNAL SQLSTATE '45000'
  SET MESSAGE_TEXT = 'User must be at least 18 years old';
END IF;

✅ Aborts the procedure and sends an error if the age is under 18.

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🔄 2. What Is RESIGNAL?

RESIGNAL is used within a handler block to re-throw the original error, optionally with a different message or SQLSTATE.

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🔹 Example – RESIGNAL with Custom Message

DECLARE CONTINUE HANDLER FOR SQLEXCEPTION
BEGIN
  RESIGNAL SET MESSAGE_TEXT = 'A database error occurred.';
END;

✅ Captures any SQL error and throws a custom message instead.

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🧪 3. Full Example – Use SIGNAL in a Stored Procedure

DELIMITER //

CREATE PROCEDURE register_user(IN username VARCHAR(50))
BEGIN
  IF username IS NULL OR username = '' THEN
    SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = 'Username cannot be empty';
  END IF;

  INSERT INTO users(name) VALUES (username);
END //

DELIMITER ;

Usage:

CALL register_user(''); -- ❌ Throws custom error
CALL register_user('Alice'); -- ✅ Success

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🔁 4. RESIGNAL Example in Error Handler

DELIMITER //

CREATE PROCEDURE risky_insert()
BEGIN
  DECLARE CONTINUE HANDLER FOR SQLEXCEPTION
  BEGIN
    RESIGNAL SET MESSAGE_TEXT = 'Error occurred during insert!';
  END;

  INSERT INTO unknown_table VALUES (1); -- Fails
END //

DELIMITER ;

✅ Customizes the message while keeping the error intact.

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📋 SIGNAL vs RESIGNAL – Comparison

Feature	SIGNAL	RESIGNAL
Raises new error	✅ Yes	❌ Only inside handler
Re-throws error	❌ No	✅ Yes, used in exception handlers
Usage area	Anywhere in code	Only inside DECLARE ... HANDLER
Custom message	✅ Via MESSAGE_TEXT	✅ Override or preserve original

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🔐 Use Cases & Benefits

Scenario	Use Case
Business rule enforcement	Prevents illegal inserts (e.g., age check)
Validation in triggers	Stops insert/update with invalid data
Custom error response to app	Sends meaningful errors (e.g., “Invalid SKU”)
Exception logging	Catch + resignal inside logging handler

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📘 Best Practices

✅ Tip	💡 Why It Matters
Always use SQLSTATE ‘45000’ for user errors	It’s the standard for custom exceptions
Avoid exposing internal errors to clients	Use RESIGNAL with safer messages
Use SIGNAL in both procedures and triggers	Allows consistent validation
Combine SIGNAL with IF logic	Helps enforce business rules easily
Keep messages short and clear	Improves debugging and user feedback

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📌 Summary – Recap & Next Steps

SIGNAL and RESIGNAL give you robust error-handling control in MySQL stored programs. Use them to build reliable, rule-aware, and user-friendly database logic.

🔍 Key Takeaways

• Use SIGNAL to raise a new error with a custom message
• Use RESIGNAL inside handlers to rethrow errors or change messages
• Always use SQLSTATE '45000' for general-purpose errors
• Combine with conditionals for data validation and error control
• Helpful for both stored procedures and triggers

⚙️ Real-World Relevance

Used in business logic enforcement, validation routines, error masking, and exception logging across real-world CRM, finance, HR, and e-commerce systems.

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❓ FAQ – MySQL SIGNAL & RESIGNAL

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❓ Can I use SIGNAL outside stored procedures?

✅ Yes. You can use it in triggers, events, or standalone scripts.

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❓ What is SQLSTATE ‘45000’?

It’s a generic code for user-defined errors in MySQL (unhandled exception).

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❓ Can I use RESIGNAL outside a handler?

❌ No. RESIGNAL is only valid inside an exception handler.

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❓ Can I specify my own SQLSTATE code?

✅ Yes, but it must be a 5-character string starting with '01'–'45' range for warnings/errors.

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❓ Can SIGNAL stop the query execution?

✅ Yes. It immediately terminates execution with the error message.

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