**What is a Trigger?**
A trigger (TRIGGER) is one of the database objects in MySQL, introduced since version 5.0.2. It functions similarly to functions in programming languages, requiring declaration and execution. However, unlike regular functions, triggers are not called directly by programs or manually; instead, they are automatically activated by specific database events. Think of it as an event listener in web development, where actions like inserting, updating, or deleting data can trigger predefined operations.
Why use triggers? In real-world applications, you often encounter scenarios where certain actions need to be performed automatically when data changes. For example:
- When adding a student to a table, the total number of students should update accordingly.
- When inserting new student records, validations for phone numbers or emails may be required.
Triggers handle these tasks automatically. They are executed when specific SQL statements—such as INSERT, UPDATE, or DELETE—are triggered. Other SQL commands do not activate them.
**MySQL Overview**
MySQL is a relational database management system developed by MySQL AB in Sweden and now owned by Oracle. It's one of the most popular RDBMS systems, especially for web applications. MySQL organizes data into multiple tables rather than storing everything in a single repository, which improves performance and flexibility.
It uses SQL (Structured Query Language), the standard language for interacting with databases. MySQL offers both a community and a commercial version. Due to its lightweight nature, fast performance, and open-source availability, it’s widely used for small and medium-sized websites.
Thanks to its strong community support, MySQL pairs well with PHP and Apache to create a powerful web development stack.
**Creating Triggers**
To create a trigger, you can use the following syntax:
```sql
CREATE [DEFINER = { user | CURRENT_USER }] TRIGGER trigger_name
trigger_time trigger_event
ON tbl_name FOR EACH ROW
trigger_body
```
- `trigger_name`: A unique name for the trigger.
- `trigger_time`: Either `BEFORE` or `AFTER`, indicating when the trigger fires relative to the event.
- `trigger_event`: The event that activates the trigger: `INSERT`, `UPDATE`, or `DELETE`.
- `tbl_name`: The table on which the trigger operates.
- `trigger_body`: The SQL statements to execute when the trigger is fired.
**Creating a Simple Trigger**
Example: Create a trigger to automatically update the student count when inserting or deleting records.
1. **Prepare the tables:**
```sql
CREATE TABLE student_info (
stu_no INT(11) NOT NULL AUTO_INCREMENT,
stu_name VARCHAR(255),
PRIMARY KEY (stu_no)
);
CREATE TABLE student_count (
student_count INT(11) DEFAULT 0
);
INSERT INTO student_count VALUES (0);
```
2. **Create the triggers:**
```sql
CREATE TRIGGER trigger_student_count_insert
AFTER INSERT ON student_info
FOR EACH ROW
UPDATE student_count SET student_count = student_count + 1;
CREATE TRIGGER trigger_student_count_delete
AFTER DELETE ON student_info
FOR EACH ROW
UPDATE student_count SET student_count = student_count - 1;
```
3. **Test the triggers:**
```sql
INSERT INTO student_info VALUES (NULL, 'Zhang Ming'), (NULL, 'Li Ming'), (NULL, 'Wang Ming');
SELECT * FROM student_count;
-- Result: 3
DELETE FROM student_info WHERE stu_name IN ('Zhang Ming', 'Li Ming');
SELECT * FROM student_count;
-- Result: 1
```
**Creating a Trigger with Multiple Statements**
If your trigger needs to perform multiple actions, wrap the code in `BEGIN...END`.
Example: Track student counts per class.
1. **Update the tables:**
```sql
ALTER TABLE student_info ADD COLUMN student_class INT;
CREATE TABLE student_count (
student_count INT(11),
student_class INT
);
INSERT INTO student_count VALUES (0, 0), (0, 1), (0, 2);
```
2. **Create the trigger:**
```sql
DELIMITER $$
CREATE TRIGGER trigger_student_count_insert
AFTER INSERT ON student_info
FOR EACH ROW
BEGIN
UPDATE student_count SET student_count = student_count + 1 WHERE student_class = 0;
UPDATE student_count SET student_count = student_count + 1 WHERE student_class = NEW.student_class;
END $$
DELIMITER ;
```
3. **Insert and test:**
```sql
INSERT INTO student_info VALUES (NULL, 'AAA', 1), (NULL, 'BBB', 1), (NULL, 'CCC', 2);
SELECT * FROM student_count;
-- Result: Total 3, Class 1 has 2, Class 2 has 1
```
**Understanding NEW and OLD Keywords**
In triggers, `NEW` refers to the new row being inserted, while `OLD` refers to the row being deleted. These keywords allow you to access values from the current or previous state of the data.
**Trigger Usage Restrictions**
- Triggers can only be created on permanent tables, not temporary ones.
- They cannot call stored procedures that return values or use dynamic SQL.
- Triggers cannot start or end transactions, but using `SAVEPOINT` is allowed.
- Foreign key operations do not trigger triggers.
- Triggers on slave tables are not activated during statement-based replication.
- Triggers cannot have a `RETURN` statement; use `LEAVE` to exit early.
**Exception Handling in Triggers**
Triggers follow a sequence: `BEFORE`, then the operation, then `AFTER`. If any step fails, the entire operation is rolled back (for transactional tables), but not for non-transactional ones.
**Viewing Triggers**
You can view all triggers using `SHOW TRIGGERS`, or query the `information_schema.triggers` table for more precise results.
**Deleting a Trigger**
To remove a trigger, use:
```sql
DROP TRIGGER trigger_name;
```
Triggers are powerful tools for automating database logic, improving consistency, and reducing manual intervention. Understanding their behavior and limitations is essential for effective database design.
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