How to create and delete MySQL triggers

MySQL How to create and delete triggers

This article mainly introduces “MySQL How to create and delete triggers ” Knowledge about , Xiaobian shows you the operation process through practical cases , The operation method is simple and fast , Practical , Hope this article “MySQL How to create and delete triggers ” The article can help you solve problems .

1. Why triggers are needed

There are some tables that are related to each other , For example, commodity table and inventory table , We operate on the data of the commodity table , Then the corresponding inventory table has to change , Only in this way can the integrity of the data be guaranteed . If we maintain it manually , More trouble .
At this point we can use triggers , Create a trigger , Let the insertion of commodity information data automatically trigger the insertion of inventory data , In this way, we don't need to worry about data loss caused by forgetting to add inventory data .

2. Trigger Overview

• MySQL It's from 5.0.2 Version starts to support trigger

• MySQL Like stored procedures, triggers are embedded in MySQL The server It's a piece of procedure

• A trigger is an event that triggers an action , These events include insert,delete,update event

• If trigger procedure is defined , When the database executes these statements , It's equivalent to an event , The trigger will be automatically fired to perform the corresponding operation

• If you are inserting data from tables in the database , When some database logic needs to be executed automatically , We can use triggers to implement .

3. Trigger creation

3.1 grammar

Triggers work on tables , For example, we want to watch A Triggered when a new record is added Trigger execution , And also choose whether the trigger is in insert Whether the trigger is executed before or after the statement is executed .

• for each row indicate , Every time an event is executed (insert,update or delete) Will trigger a trigger

CREATE TRIGGER  Trigger Name  {BEFORE|AFTER} {INSERT|UPDATE|DELETE} ON  Table name  FOR EACH ROW  The block of statements executed by the trigger ;

explain ：

• Table name ： Represents the object monitored by the trigger .

• BEFORE|AFTER： Indicates the trigger time .BEFORE Indicates that... Is triggered before the event ;AFTER Indicates that... Is triggered after the event .

• INSERT|UPDATE|DELETE： Indicates the event triggered .

• INSERT Indicates that... Is triggered when a record is inserted ;

• UPDATE Indicates that... Is triggered when the record is updated ;

• DELETE Indicates that... Is triggered when a record is deleted .

• The block of statements executed by the trigger ： It can be a single SQL sentence , It can also be done by BEGIN…END Structure .

3.2 Case presentation

Prepare the table first

CREATE TABLE test_trigger (id INT PRIMARY KEY AUTO_INCREMENT,t_note VARCHAR(30));CREATE TABLE test_trigger_log (id INT PRIMARY KEY AUTO_INCREMENT,t_log VARCHAR(30));

requirement : Create trigger ： Create name as before_insert The trigger of , towards test_trigger Data table before inserting data , towards test_trigger_log Insert... Into the data table before_insert Log information .

4. Check triggers

• View trigger is to view the definition of triggers that already exist in the database , Status and syntax information

• Mode one : View the definitions of all triggers in the current database

show triggers\G  Be careful , stay SQLyog in , It can't be added \G

• Mode two ： View the definition of a trigger in the current database

show create trigger  Trigger Name 

• Mode three : From the system library information_schema Of TRIGGERS Query in table “salary_check_trigger” Trigger information

SELECT * FROM information_schema.TRIGGERS;

5. Delete trigger

Triggers are also database objects , Triggers are also used drop Statement to delete

drop trigger if exists  Trigger Name ;

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