MySQL (17 trigger)

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One Trigger Overview

• MySQL from 5.0.2 Trigger is supported in version .MySQL The trigger is the same as the stored procedure , Are embedded in MySQL A program of the server .
• The trigger is made by Events to trigger Some operation , These events include INSERT 、 UPDATE 、 DELETE event . The so-called event refers to the user's action or triggering a certain behavior . If the trigger program is defined , When the database executes these statements , It's equivalent to an event , will Automatically The trigger performs the corresponding action .
• When inserting data in the data table 、 Update and delete operations , When some database logic needs to be executed automatically , Triggers can be used to implement .

Two Trigger creation

2.1 grammar -‘ Only child ’

• Grammatical structure

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 （ Relative to event ）.
  • INSERT|UPDATE|DELETE： Indicates the departure event
    • INSERT —— Triggered when a record is inserted ;
    • UPDATE —— Triggered when the record is updated .
    • DELETE —— Triggered when a record is deleted .
  • The block of statements executed by the trigger ： It can be a single sql sentence , Or by BEGIN……END Structure .

2.2 Case understanding -‘ triplets ’

🪬🪬2.2.1 Case a

• 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 .

DELIMITER //
CREATE TRIGGER before_insert
BEFORE INSERT ON test_trigger
FOR EACH ROW
BEGIN
INSERT INTO test_trigger_log (t_log)
VALUES('before_insert');
END //
DELIMITER ;

2.2.2 Case 2

• Create name as after_insert The trigger of , towards test_trigger After inserting data into the data table , towards test_trigger_log Insert in data table
  Enter into after_insert Log information

DELIMITER //
CREATE TRIGGER after_insert
AFTER INSERT ON test_trigger
FOR EACH ROW
BEGIN
INSERT INTO test_trigger_log (t_log)
VALUES('after_insert');
END //
DELIMITER ;

2.2.3 Case three

DELIMITER //
CREATE TRIGGER salary_check_trigger
BEFORE INSERT ON employees FOR EACH ROW
BEGIN
DECLARE mgrsalary DOUBLE; 
SELECT salary INTO mgrsalary FROM employees WHERE employee_id = NEW.manager_id;
IF NEW.salary > mgrsalary THEN #NEW Keyword represents INSERT Add a new record of the statement 
SIGNAL SQLSTATE 'HY000' SET MESSAGE_TEXT = ' The salary is higher than the leader's salary ';
END IF;
END //
DELIMITER ;

3、 ... and see 、 Delete trigger

3.1 View by -‘ Three brothers ’

• The eldest brother – View the definitions of all triggers in the current database

SHOW TRIGGERS\G

• The second – View the definition of a trigger in the current database

SHOE CREATE TRIGGER  Trigger Name 

• Youngest – From the system library information_schema Of TRIGGERS Query in table “salary_check_trigger” Trigger information .

SELECT * FROM information_schema.TRIGGERS;

3.2 Delete trigger

• Triggers are also database objects , Delete triggers also use DROP sentence , The syntax is as follows ：

DROP TRIGGER IF EXISTS  Trigger Name ;

Four Success and failure of triggers

4.1 become

1. Triggers ensure data integrity
2. Triggers can help us log operations .
3. Triggers can also be used before manipulating data , Check the validity of the data

🧩🧩4.2 Defeat

1. One of the biggest problems with triggers is poor readability .
2. Change of relevant data , May cause trigger errors