MySQL trigger

Trigger Overview

Basic concepts

MySQL As if from 5.0.2 The version has been supporting the function of triggers .
Triggers are database objects related to tables , Triggered when the defined conditions are met , And execute the set of statements defined in the trigger . This feature of trigger can help to ensure the integrity of data in database .

The characteristics of triggers ：

1、 Yes begin end body ,begin end; The statements between can be simple or complex

2、 What conditions trigger ：insert,update,delete

3、 When to trigger ： Before or after the addition, deletion or modification

4、 Trigger frequency ： Execute... For each line

5、 Triggers are defined on the table , Attach to table .

Trigger execution takes time , Therefore, it is not recommended to use triggers for data tables with frequent additions, deletions, and modifications , Because it reduces efficiency

The type of trigger

According to the trigger time of the trigger and the triggered operation, the trigger is divided into 6 Type in the

• 1 BEFORE INSERT
• 2 AFTER INSERT
• 3 BEFORE UPDATE
• 4 AFTER UPDATE
• 5 BEFORE DELETE
• 6 AFTER DELETE

Create trigger Syntax

CREATE
    [DEFINER = { user | CURRENT_USER }]
TRIGGER trigger_name
trigger_time trigger_event
ON tbl_name FOR EACH ROW
　　[trigger_order]
trigger_body

trigger_time: { BEFORE | AFTER }

trigger_event: { INSERT | UPDATE | DELETE }

trigger_order: { FOLLOWS | PRECEDES } other_trigger_name

BEFORE and AFTER Parameter specifies when execution is triggered , Before or after the event .

FOR EACH ROW Indicates that the trigger event will be triggered if the operation on any record satisfies the trigger event , In other words, the trigger frequency of the trigger is triggered once for each row of data .

tigger_event Detailed explanation ：

①INSERT Type trigger ： Activate trigger when inserting a row , May be through INSERT、LOAD DATA、REPLACE Statement trigger (LOAD DAT Statement is used to load a file into a data table , It's equivalent to a series of INSERT operation );

②UPDATE Type trigger ： Activate trigger when changing a row , May be through UPDATE Statement trigger ;

③DELETE Type trigger ： Activate trigger when deleting a row , May be through DELETE、REPLACE Statement trigger .

trigger_order yes MySQL5.7 The next function , Used to define multiple triggers , Use follows( Follow ) or precedes( stay … prior to ) To select the sequence of trigger execution .

Hard vegetable

BEFORE INSERT

stay BEFORE INSERT Trigger NEW Indicates adding a new row of data , have access to NEW.xxx To get a column of data

DELIMITER $$
CREATE TRIGGER triger1 BEFORE INSERT
ON book FOR EACH ROW
BEGIN
	DECLARE v_title VARCHAR(255);
	if length(NEW.title)>5 THEN
		SET NEW.title = ' The title is too long , So just cut it off ';
	end if;
END $$
DELIMITER ;

test ：

insert into book (isbn,title,price,detail,pubTime,authorId) 
	values('95275862',' The title is long ',125,' It is used to test triggers ','2021-9-25',1)

AFTER INSERT

there NEW Or the newly added data row

DELIMITER $$
CREATE TRIGGER triger2 AFTER INSERT
ON book FOR EACH ROW
BEGIN
	insert into temp values(NEW.bookid);
END $$
DELIMITER ;

BEFORE UPDATE

there NEW Represents a new data row . OLD Represents the data row to be modified .

DELIMITER $$
CREATE TRIGGER triger3 BEFORE UPDATE
ON book FOR EACH ROW
BEGIN
			insert into temp values(CONCAT(' Add the title :',OLD.title,' It is amended as follows :',NEW.title));
END $$
DELIMITER ;

test ：

update book set title=' short ' where isbn = '95275863';

AFTER UPDATE

It is completely consistent with the description and writing of the previous case

BEFORE DELETE

Nothing here NEW, Only OLD,OLD It means the data row to be deleted

DELIMITER $$
CREATE TRIGGER triger4 BEFORE DELETE
ON book FOR EACH ROW
BEGIN
			insert into temp values(CONCAT(' Ready to delete :',OLD.title));
END $$
DELIMITER ;

AFTER DELETE

It is completely consistent with the description and writing of the previous case