Trigger definition and syntax introduction in MySQL

Catalog

1. Definition ：

2. grammar ：

3. Delete trigger

4. Query triggers

5. Trigger Type OLD and NEW Use

1. Definition ：

　　 Triggers are similar to stored procedures , Are embedded in MySQL One of the procedures in . A trigger is an action triggered by an event . When the database executes these events , The trigger will be activated to perform the corresponding operation . These events are called trigger conditions , stay MySQL in , Yes UPDATE,INSERT, and DELETE.

stay MySQL in , At present, only UPDATE,INSERT, and DELETE These three operations , Trigger will be triggered , Triggers are not supported for other operations . This sum Oracle The triggers in are very different , Pay attention to the distinction .

Each table of the trigger each event , Only one trigger is allowed . therefore , The maximum value allowed for each table is 6 Trigger . If a table has two events ,INSERT and DELETE, Then create a trigger respectively .

Only tables support triggers , Views and temporary tables do not support .

2. grammar ：CREATE TRIGGER TRIGGER_NAMEBEFORE/AFTER UPDATE/INSERT/DELETE ON TABLE_NAME FOR EACH ROW# Trigger body [BEGIN]SQL operation [END];

Create trigger ： It's the same as creating a stored procedure , Using all of these CREATE sentence , But should pay attention to , Nothing here OR REPLACE.

TRIGGER_NAME： Trigger Name . stay MySQL in , Ensure that the triggers in each table are unique , Triggers in each database may not be unique , This shows that two tables in a database can have the same trigger . This behavior is not allowed in other databases . But for the sake of standard format , Still try to guarantee TRIGGER_NAME Uniqueness .

BEFORE/AFTER： When the trigger is triggered . If you want the trigger to happen before the event is triggered , Just use BEFORE, If you want to happen after the trigger event , Just use AFTER.

UPDATE/INSERT/DELETE： Triggering event , The trigger condition . stay MySQL in , There are only three DDL Actions support triggers , Other operations are not allowed .

FOR EACH ROW： Each row affected by the trigger event must activate the trigger action . This refers to row level triggering , stay MySQL in ,FOE EACH ROW Do not omit , Statement level triggering is not supported .

Trigger body ： It can be a single one SQL sentence , It can also be done by BEGIN...END A complex structural block composed of .

# Create a trigger T1, On the table customer When inserting , It will trigger T1, Then it will be in the log table note Insert a piece of data in CREATE TRIGGER T1AFTER INSERT ON customerFOR EACH ROW # Trigger body INSERT INTO note( date , The goal is , operation ) VALUES(NOW(),'customer','insert');3. Delete trigger

　　 Similar to deleting tables and stored procedures , Is to use DROP Statement to delete the trigger .

# Delete trigger T1DROP TRIGGER IF EXISTS T1;

Triggers cannot be updated or modified , Want to modify the contents of a trigger , The trigger can only be deleted first , Creating a new trigger .

4. Query triggers # In a known database , Query triggers SHOW TRIGGERS;SELECT * FROM information_schema.TRIGGERS WHERE TRIGGER_NAME = ' Trigger Name '# Query all triggers SELECT * FROM information_schema.TRIGGERS（WHERE TRIGGER_SCHEMA = ' Database name '）5. Trigger Type OLD and NEW Use

　　 Trigger in DDL Triggered before or after the operation , Always make some changes to the target table , Sometimes we need to know the values before and after the target table changes , And that's where it comes in OLD and NEW Usage of .

seeing the name of a thing one thinks of its function ,OLD Is to change the previous value , yes UPDATE Before or to be DELETE Or has been DELETE Value .

　　 and NEW, It is UPDATE The new value after , the INSERT Or already INSERT Value .

　　 Specific examples ：

# establish customer surface CREATE TABLE `customer` (`CUST_ID` int(11) NOT NULL AUTO_INCREMENT,`CUST_NAME` varchar(10) NOT NULL,`CUST_TEL` varchar(10) DEFAULT NULL,PRIMARY KEY (`CUST_ID`)) ENGINE=InnoDB DEFAULT CHARSET=utf8;# Insert data into table # Create a update trigger CREATE TRIGGER T2AFTER UPDATE ON customerFOR EACH ROW #old and new How to use ：old.columnname/new.columnname（ Name ）# The values before and after will be updated , Assign to two variables SELECT OLD.CUST_ID,NEW.CUST_ID INTO @OLD_ID,@NEW_ID;# Trigger trigger T2UPDATE customer SET CUST_ID = '10000' WHERE CUST_NAME = 'AAA';# Inquire about OLD and NEWSELECT @OLD_ID,@NEW_ID;

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