Introduction to SQL Triggers – What They Are and How They Work

Introduction

In SQL, text data types are used to store alphanumeric values like names, addresses, emails, and descriptions. Choosing the correct text type — CHAR, VARCHAR, or TEXT — is important for optimizing storage space, query speed, and database performance.

In this section, you'll learn the definitions, differences, and best use cases for each text data type.

1. CHAR (Fixed-Length String)

CHAR is used to store fixed-length strings. If the stored string is shorter than the defined length, SQL automatically pads it with spaces to match the specified size.

Features:

• Fixed length
• Fast and predictable performance
• Uses extra storage if the data is often shorter than the specified length

Syntax:

column_name CHAR(length);

length = number of characters (1 to 255 depending on the database system)

Example:

CREATE TABLE countries (
country_code CHAR(2),
country_name CHAR(50)
    );

country_code like 'US', 'IN', 'UK' will always take 2 characters.

When to Use CHAR:

• Data with a constant size, such as country codes, gender ('M', 'F'), state abbreviations
• Fixed-format fields like credit card types ('VISA', 'MC')
• When exact storage size is known and consistent

2. VARCHAR (Variable-Length String)

VARCHAR stands for Variable Character. It stores variable-length strings, meaning only the actual characters are stored without unnecessary padding.

Features:

• Variable length
• More space-efficient than CHAR for varying-length text
• Slightly slower than CHAR when processing large volumes (because of extra calculations for string lengths)

Syntax:

column_name VARCHAR(length);

length = maximum number of characters allowed

Example:

CREATE TABLE employees (
first_name VARCHAR(50),
email VARCHAR(100)
    );

Names and emails can vary in length, making VARCHAR ideal.

When to Use VARCHAR:

• Data with unpredictable or variable length
• Names, emails, addresses, and descriptions under 255-65535 characters
• Most general-purpose text fields

3. TEXT (Large Text Field)

TEXT is used to store large amounts of text like long descriptions, blog posts, comments, or articles.

Features:

• Meant for large text storage (up to 65,535 characters for standard TEXT in MySQL)
• Cannot have a default value (in some databases like MySQL)
• TEXT fields are stored outside the main table with a pointer reference
• Different variants exist (TINYTEXT, MEDIUMTEXT, LONGTEXT) for various sizes

Syntax:

column_name TEXT;

Example:

CREATE TABLE articles (
id INT,
title VARCHAR(255),
body TEXT
   );

body will store the full article content, which can be very large.

When to Use TEXT:

• Long-form text fields (comments, articles, reviews, reports)
• Data that exceeds normal VARCHAR limits
• When exact storage requirements are unknown or potentially very large

Quick Comparison: CHAR vs VARCHAR vs TEXT

Important Tips

• Use CHAR only when all values will be exactly the same length
• VARCHAR is the best choice for most standard text fields
• Reserve TEXT for content that exceeds VARCHAR limits
• Consider VARCHAR(MAX) in SQL Server for large text that might need indexing
• Be aware that TEXT fields may have limitations on default values and full-text indexing

What Is a Trigger in SQL?

A trigger is a special kind of stored program in SQL that automatically executes in response to certain events on a table. These events can be:

• INSERT
• UPDATE
• DELETE

Triggers help enforce business rules, maintain data integrity, and automate database operations without manual intervention.

Why Use Triggers?

Triggers are useful for:

• Auditing changes to a table (e.g., who updated what and when)
• Automatically updating related tables
• Enforcing complex constraints beyond standard SQL rules
• Logging and monitoring data changes

How SQL Triggers Work

A trigger is tied to a table and is executed:

• BEFORE or AFTER a specific operation (INSERT, UPDATE, DELETE)
• On each row affected by the operation

Trigger Timing Options:

Basic Trigger Syntax (MySQL)

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

Example 1: AFTER INSERT Trigger

Automatically logs new records to an audit table:

CREATE TRIGGER after_customer_insert
AFTER INSERT ON customers
FOR EACH ROW
BEGIN
   INSERT INTO customer_audit(customer_id, action, created_at)
   VALUES (NEW.id, 'INSERT', NOW());
END;

Example 2: BEFORE UPDATE Trigger

Prevents negative salary updates:

CREATE TRIGGER before_salary_update
BEFORE UPDATE ON employees
FOR EACH ROW
BEGIN
   IF NEW.salary < 0 THEN
     SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Salary cannot be negative';
   END IF;
END;

OLD and NEW in Triggers

• OLD: Refers to the value before the change (used in UPDATE and DELETE)
• NEW: Refers to the value after the change (used in INSERT and UPDATE)

Key Points

• Triggers cannot return a value
• They are automatically fired; no manual execution is needed
• Use triggers wisely – they can make debugging and performance tuning more difficult if overused

Best Practices

• Use clear and specific trigger names
• Avoid complex logic inside triggers
• Keep triggers short and efficient
• Document your triggers properly