How to use triggers in MySQL? Detailed explanation of MySQL triggers
MySQL is a commonly used relational database management system that supports the use of triggers. A trigger is a special stored procedure in MySQL that can execute a series of SQL statements on a specific table, similar to the event-driven programming model. Use triggers to implement complex business logic and improve database performance and reliability. This article will introduce in detail how to use triggers in MySQL .
1. Create a trigger
In MySQL , triggers can be created using the CREATE TRIGGER statement. The basic syntax of the CREATE TRIGGER statement is as follows:
CREATE TRIGGER trigger_name trigger_time trigger_event ON table_name FOR EACH ROW trigger_body;
Among them, trigger_name is the name of the trigger; trigger_time is the execution time of the trigger, which can be BEFORE or AFTER ; trigger_event is the event of the trigger, which can be INSERT , UPDATE or DELETE ; table_name is the name of the table where the trigger is located; trigger_body is the trigger The SQL statement of the server .
For example, the following trigger creates a BEFORE INSERT trigger on the employee table , which automatically sets the entry time to the current time when a new employee joins:
CREATE TRIGGER set_join_date BEFORE INSERT ON employee FOR EACH ROW
BEGIN
SET NEW.join_date = NOW();
END;
2. Trigger execution time
In MySQL , triggers can execute before or after an INSERT , UPDATE , or DELETE event. The execution time of a trigger can be BEFORE or AFTER .
BEFORE trigger
The BEFORE trigger is executed before performing an INSERT , UPDATE or DELETE operation, and can implement some data preprocessing and verification in the database.
For example, the following trigger creates a BEFORE UPDATE trigger on the employee table . When the employee's salary is greater than 10,000 , his salary is set to 10,000 :
CREATE TRIGGER limit_salary BEFORE UPDATE ON employee FOR EACH ROW
BEGIN
IF NEW.salary > 10000 THEN
SET NEW.salary = 10000;
END IF;
END;
AFTER trigger
The AFTER trigger is executed after performing an INSERT , UPDATE or DELETE operation, and can implement some data post-processing and calculations in the database.
For example, the following trigger creates an AFTER INSERT trigger on the order table , which automatically updates the product's inventory when a new order is added:
CREATE TRIGGER update_inventory AFTER INSERT ON order FOR EACH ROW
BEGIN
UPDATE product SET inventory = inventory - NEW.quantity WHERE id = NEW.product_id;
END;
3. Trigger events
In MySQL , triggers can execute on INSERT , UPDATE or DELETE events. The INSERT event is triggered when a new record is inserted, the UPDATE event is triggered when a record is updated, and the DELETE event is triggered when a record is deleted. Triggers can execute on multiple events.
For example, the following trigger creates a BEFORE INSERT or UPDATE trigger on the employee table, and when the employee's salary is greater than 10,000 , his salary is set to 10,000 :
CREATE TRIGGER limit_salary BEFORE INSERT OR UPDATE ON employee FOR EACH ROW
BEGIN
IF NEW.salary > 10000 THEN
SET NEW.salary = 10000;
END IF;
END;
4. Trigger execution conditions
In MySQL , you can use the IF statement to control the execution conditions of triggers. The IF statement can determine whether to execute the SQL statement in the trigger based on conditions .
For example, the following trigger creates a BEFORE INSERT trigger on the employee table . When the employee's salary is less than 5,000 , his entry time is set to the current time:
CREATE TRIGGER set_join_date BEFORE INSERT ON employee FOR EACH ROW
BEGIN
IF NEW.salary < 5000 THEN
SET NEW.join_date = NOW();
END IF;
END;
5. Trigger process control
In MySQL , triggers can use flow control statements to control the execution flow of the program, such as IF statements, CASE statements, WHILE statements, LOOP statements, etc. Flow control statements can help developers implement more complex business logic.
For example, the following trigger creates a BEFORE INSERT trigger on the employee table . When the employee's salary is less than 5,000 , his joining time is set to the current time, otherwise his joining time is set to the time one month ago:
CREATE TRIGGER set_join_date BEFORE INSERT ON employee FOR EACH ROW
BEGIN
IF NEW.salary < 5000 THEN
SET NEW.join_date = NOW();
ELSE
SET NEW.join_date = DATE_SUB(NOW(), INTERVAL 1 MONTH);
END IF;
END;
6. Optimization of triggers
Triggers can help developers optimize database performance and reliability. Here are some trigger optimization tips:
Minimize the execution time of triggers and avoid triggers becoming large and complex.
Avoid executing complex SQL statements in triggers, as complex SQL statements will cause performance degradation.
Use trigger execution conditions and flow control statements to control the execution flow and avoid unnecessary execution of triggers.
Avoid executing multiple SQL statements in triggers , as multiple SQL statements will cause performance degradation.
7. Summary
A trigger is a special stored procedure in MySQL that can execute a series of SQL statements on a specific table, similar to the event-driven programming model. This article introduces how to create triggers in MySQL , control trigger execution time, event and process control, optimize triggers, etc. Developers can flexibly use triggers according to needs and scenarios to improve the efficiency and reliability of the MySQL database.