MySQL triggers

• 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 , however , Trigger does not need to be called , When the execution in the database table DML This is triggered automatically during operation SQL Execution of fragments , No manual call required .
• stay MySQL, 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 apply the changed records in the trigger , This is similar to other databases . Now triggers only support row level triggering , Statement level triggering is not supported .

The characteristics of triggers ：

1. What conditions trigger ：I、D、U
2. When to trigger ： Before or after addition, deletion and modification
3. Trigger frequency ： Execute... For each line
4. Triggers are defined on the table , Attach to table

Basic operation

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;

for example ：

--  Data preparation 
CREATE DATABASE mydb10_trigger;
USE mydb10_trigger;

--  User table 
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)
);

--  Need one ： When user Add a row of data to the table , Will be automatically in user_log Add logging 
--  Defining triggers ：trigger_test1
CREATE TRIGGER trigger_test1 AFTER INSERT
ON user FOR EACH ROW
INSERT INTO user_logs VALUES (NULL,NOW(),' New users added ');

--  stay user Table add data , Let the trigger execute automatically 
INSERT INTO user VALUES(1,' Zhang San ','123456');
INSERT INTO user VALUES(2,' Li Si ','234567'),(3,' Wang Wu ','345678');

--  Demand two ： When user Table data is modified , Will be automatically in user_log Add logging 
DELIMITER $$
CREATE TRIGGER trigger_test2 BEFORE UPDATE
ON user FOR EACH ROW
BEGIN
INSERT INTO user_logs VALUES (NULL,NOW(),' User information has been modified ');
END $$
DELIMITER ;

--  stay user Table modify data , Let the trigger execute automatically 
UPDATE user SET password = '888888' WHERE uid = 1;
UPDATE user SET password = '666666' WHERE uid = 1;

NEW and OLD

Concept ：MySQL It defines NEW and OLD, Used to represent the table where the trigger is located , The row that triggered the trigger , To refer to the changed record content in the trigger , Concrete ：

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 Represents the data that will be or has been modified
DELETE Type trigger	OLD Data that will be or has been deleted

Method ：NEW.columnName (columnName A column name for the corresponding data table )

--  Data preparation 
CREATE DATABASE mydb10_trigger;
USE mydb10_trigger;

--  User table 
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)
);

--  Need one ： When user Add a row of data to the table , Will be automatically in user_log Add logging 
--  Defining triggers ：trigger_test1
CREATE TRIGGER trigger_test1 AFTER INSERT
ON user FOR EACH ROW
INSERT INTO user_logs VALUES (NULL,NOW(),' New users added ');

--  stay user Table add data , Let the trigger execute automatically 
INSERT INTO user VALUES(1,' Zhang San ','123456');
INSERT INTO user VALUES(2,' Li Si ','234567'),(3,' Wang Wu ','345678');

--  Demand two ： When user Table data is modified , Will be automatically in user_log Add logging 
DELIMITER $$
CREATE TRIGGER trigger_test2 BEFORE UPDATE
ON user FOR EACH ROW
BEGIN
INSERT INTO user_logs VALUES (NULL,NOW(),' User information has been modified ');
END $$
DELIMITER ;

--  stay user Table modify data , Let the trigger execute automatically 
UPDATE user SET password = '888888' WHERE uid = 1;
UPDATE user SET password = '666666' WHERE uid = 1;


-- INSERT Type of trigger 
-- NEW
--  Defining triggers ：trigger_test3
CREATE TRIGGER trigger_test3 AFTER INSERT
ON user FOR EACH ROW
INSERT INTO user_logs VALUES (NULL,NOW(),CONCAT(' New users added , The message is ：',NEW.uid,NEW.username,NEW.password));
INSERT INTO user VALUES(4,' Zhao Liu ','234567');

-- UPDATE Type of trigger 

-- OLD
CREATE TRIGGER trigger_test4 AFTER UPDATE
ON user FOR EACH ROW
INSERT INTO user_logs VALUES(NULL,NOW(),CONCAT(' User information modification , The information was modified to ：',OLD.uid,OLD.username,OLD.password));

UPDATE user SET password ='9999999' WHERE uid=4;

-- NEW
CREATE TRIGGER trigger_test5 AFTER UPDATE
ON user FOR EACH ROW
INSERT INTO user_logs VALUES(NULL,NOW(),CONCAT_WS(',',' User information modification , The information is modified to ：',NEW.uid,NEW.username,NEW.password));

UPDATE user SET password ='000000' WHERE uid=1;

-- DELETE Type of trigger 

-- OLD
CREATE TRIGGER trigger_test6 AFTER DELETE
ON user FOR EACH ROW
INSERT INTO user_logs VALUES(NULL,NOW(),CONCAT_WS(',',' User information has been deleted , The deleted user information is ：',OLD.uid,OLD.username,OLD.password));

DELETE FROM user WHERE uid=4;

Other operating

Check triggers

Method ：
SHOW triggers;

Delete trigger

Method ：
DROP TRIGGER [IF EXISTS] trigger_name;

for example ：

--  Check triggers 
SHOW TRIGGERS;

--  Delete trigger 
DROP TRIGGER IF EXISTS trigger_test6;

matters needing attention

1. MySQL The trigger in cannot insert,update,delete operation , To prevent recursive loops from triggering .
2. Use as few triggers as possible , Suppose the trigger triggers each execution 1s,insert table 500 Data , Then you need to trigger 500 trigger , Just the time it takes for the trigger to execute 500s, and insert 500 The total number of pieces of data is 1s, that insert The efficiency of is very low .
3. Triggers are for each row ; Do not use triggers on tables with frequent additions, deletions, and changes , Because it will consume resources very much .