In-depth understanding of MySQL trigger parameter settings

MySQL trigger is a series of operations defined in a database table that automatically triggers execution when specific conditions are met. Triggers can execute some specific SQL statements before or after insert, update or delete operations to achieve automated processing when data changes. The parameter settings of triggers are very important for correct usage and efficiency optimization. This article will delve into the parameter settings of MySQL triggers and analyze them with specific code examples.

1. Basic syntax of triggers

In MySQL, creating a trigger mainly includes the following parts:

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

The trigger can be executed before or after the insert, update or delete operation, or it can be executed for each row of records, ensuring that each operation will trigger the corresponding SQL statement.

2. Trigger parameter settings

2.1 Parameter description

When creating a trigger, you can set the following parameters:

• trigger_name: The name of the trigger, which must be unique in the database.
• {BEFORE | AFTER}: The time point at which the trigger is executed, which can be executed before (BEFORE) or after (AFTER) the operation.
• {INSERT | UPDATE | DELETE}: The operation type associated with the trigger, which can be insert, update or delete.
• ON table_name: The table name associated with the trigger.
• FOR EACH ROW: Determines whether the trigger is executed for each row record or for each operation.

2.2 Parameter Example

The following uses a specific example to illustrate the setting of trigger parameters:

Suppose there is a student table (students) and a grades table (scores), the student table contains basic information of students, the score table records students’ test scores, and the two tables are related through student IDs.

Now we want to automatically update the total score and average score of the corresponding students in the student table when inserting or updating the score table. We can achieve this function by creating a trigger:

DELIMITER //
CREATE TRIGGER update_student_avg_score
AFTER INSERT ON scores
FOR EACH ROW
BEGIN
    DECLARE total_score INT;
    DECLARE avg_score DECIMAL(5,2);
    
    SELECT SUM(score) INTO total_score FROM scores WHERE student_id = NEW.student_id;
    
    SET avg_score = total_score / (SELECT COUNT(*) FROM scores WHERE student_id = NEW.student_id);
    
    UPDATE students
    SET total_score = total_score NEW.score, avg_score = avg_score
    WHERE student_id = NEW.student_id;
END;
//
DELIMITER ;

In the above example, we created a trigger named update_student_avg_score, which is executed after inserting scores into the score table. Each time a grade record is inserted, the trigger calculates the student's total score and average score, and updates the corresponding student's data in the students table.

3. Summary

Through this article's in-depth understanding of the parameter settings of MySQL triggers, we can use triggers more flexibly to automate database operations. In practical applications, rationally setting trigger parameters can improve the efficiency and reliability of the system and reduce repetitive operations. We hope that through the introduction and sample code of this article, readers can better master the skills of using MySQL triggers and improve the efficiency and accuracy of database operations.

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