[MySQL] detailed explanation of trigger content of database advanced

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This article is from the column ：MySQL8.0 Learning notes
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1 Trigger Overview

Trigger introduction ：

• trigger , Is a special stored procedure . A trigger, like a stored procedure, is a function that can perform a specific function , Stored on the database server SQL fragment . But the trigger does not need to call , When executing DML This will be triggered automatically during operation SQL Execution of fragments , No manual call required .
• stay MySQL in , Only execute insert、delete、update The execution of the trigger can only be triggered when the operation ;
• This feature of trigger can help to ensure the integrity of data in database , logging , Data verification and other operations ;
• Use the alias OLD And NEW To reference the record content where the trigger changes , This is similar to other databases . Now the trigger is still Only row level triggering is supported , Statement level triggering is not supported .

Schematic diagram of trigger ：

Triggers are defined on the table , Attach to table .
Triggers are defined , You need to specify the ：

1. What conditions trigger ？ Insert 、 Delete 、 to update ？
2. When to trigger ？ Before or after addition, deletion and modification ？
3. Trigger frequency , For each line .

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2 The basic operation of trigger

2.1 Create trigger

1. Create a trigger with only one execution statement ：

create trigger  Trigger Name  before|after  Triggering event 
on  Table name  for each row
 Execute statement ;

2. Create a trigger with multiple execution statements ：

create trigger  Trigger Name  before|after  Triggering event 
on  Table name  for each row
begin
	 Execute statement list 
end;

2.2 Trigger operation instance

First, prepare the data , Define two tables user And user_logs, Record the user registration information and user operation log respectively . Hope to be user When something changes ,user_logs Change automatically （ It is realized by trigger ）. The relevant codes for data preparation are as follows ：

CREATE DATABASE IF NOT EXISTS mydatabase_tigger;
USE mydatabase_tigger;

--  User table creation 
CREATE TABLE user(
    uid INT PRIMARY KEY ,
    username VARCHAR(50) NOT NULL ,
    password VARCHAR(50) NOT NULL
);

--  User information operation log table 
CREATE TABLE user_logs(
    id INT PRIMARY KEY AUTO_INCREMENT,
    time TIMESTAMP,
    log_text VARCHAR(255)
);

• If you want to give user When adding information to the table ,user_logs The table automatically adds a row of data , A trigger is defined in the following code trigger_test01 To meet the needs , The relevant codes and results are as follows ：

CREATE TRIGGER trigger_test01
    AFTER INSERT
    ON user
    FOR EACH ROW
    INSERT INTO user_logs
    VALUES (NULL, NOW(), ' New users add ');

INSERT INTO user VALUES (1, 'nezuko', '123456');

• When user When the table data is modified , Automatically in user_logs Add logging ., A trigger is defined in the following code trigger_test02 To meet the needs , The relevant codes and results are as follows ：

DELIMITER $$
CREATE TRIGGER trigger_test02
    BEFORE UPDATE
    ON user
    FOR EACH ROW
BEGIN
    INSERT INTO user_logs VALUES (NULL, NOW(), ' User information is modified ');
END $$
DELIMITER ;

UPDATE user SET password = '111111' WHERE uid = 1;

You can see user The password in the table has been changed , The record of modification information is successfully added to the log table .

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3 NEW And OLD

3.1 Why NEW And OLD?

MySQL It defines NEW and OLD, Used to indicate that the trigger is in the table , Which row of data triggered the trigger , To refer to the changed record content in the trigger , See the following table for details ：

Trigger Type	Trigger Type NEW And OLD Use
INSERT Type trigger	NEW Indicates the data to be added or added
UPDATE Type trigger	OLD Represents the data before modification ,NEW Indicates the data to be or have been modified
DELETE Type trigger	OLD Data that will be or has been deleted

Usage method ：
NEW.columnName(columnName A column name representing the corresponding data )

What kind of requirements will be used NEW and OLD Well ？

In the case illustrated above , We modify user The password in the table , to user_logs Modification information is added to the table . But it only prompts the modified information , There is no indication that . If , It is necessary to indicate the password before modification or the password after modification in the information , You need to use the OLD and NEW Used for triggers to reference data ！

3.2 NEW And OLD example

Defining triggers trigger_test03, You can give user_logs Insert modification information , Show the user's password before and after modification . The relevant codes and results are as follows ：

DELIMITER $$
CREATE TRIGGER trigger_test03
    BEFORE UPDATE
    ON user
    FOR EACH ROW
BEGIN
    INSERT INTO user_logs VALUES (NULL, NOW(), CONCAT(' User information is modified , The password before modification is ', OLD.password, ', Modified for ', NEW.password));
END $$
DELIMITER ;

UPDATE user SET password = '99999999' WHERE uid = 1;

If you want to delete the trigger , You can use the following statement ： Delete the three triggers created so far .

DROP TRIGGER IF EXISTS trigger_test01;
DROP TRIGGER IF EXISTS trigger_test02;
DROP TRIGGER IF EXISTS trigger_test03;

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4 Other operations of trigger

1. Check triggers ：

show triggers;

2. Delete trigger ：

drop trigger if exists trigger_name;

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5 Precautions for trigger

• MySQL This table cannot be insert、update、delete operation , To prevent recursive loops from triggering ;
• Use as few triggers as possible , Suppose the trigger triggers each execution 1s, Then modify the table every time 、 The update operation will take some extra time , Thus, the efficiency of table operation is low ;
• Triggers are for each row , Try not to use triggers for tables that are frequently added, deleted, or modified , Avoid additional consumption of resources ;
• Frequent use of triggers will lead to more trouble in maintaining data in the future , Because there are many behaviors in one change .

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At the end

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