MySQL - CREATE TRIGGER

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Triggers are generally defined as responses to an event. For instance, when we hover a mouse-pointer on a drop-down menu of a website, a various set of options to navigate through this website are then displayed. Here, the hovering of the mouse-pointer is an event while the display of options in the drop-down menu is a result of trigger execution. This concept is also introduced in MySQL.

Triggers in MySQL are stored programs similar to procedures. These can be created on a table, schema, view and database that are associated with an event and whenever an event occurs the respective trigger is invoked.

Triggers are, in fact, written to be executed in response to any of the following events −

  • A database manipulation (DML) statement (DELETE, INSERT, or UPDATE)
  • A database definition (DDL) statement (CREATE, ALTER, or DROP).
  • A database operation (SERVERERROR, LOGON, LOGOFF, STARTUP, or SHUTDOWN).

Creating Trigger in MySQL

You can create a trigger using the CREATE TRIGGER Statement.

Syntax

Following is the syntax of the MySQL CREATE TRIGGER Statement.

CREATE TRIGGER trigger_name
trigger_time trigger_event
ON table_name FOR EACH ROW
BEGIN
...
END;

Where,

  • trigger_name is the name of the trigger you need to create
  • trigger_time is the time of trigger activation
  • trigger_event can be INSERT, UPDATE, or DELETE. This event causes the trigger to be invoked.
  • table_name is the name of the table to which the trigger is associated with.

Example

Assume we have created a table with name student as shown below −

CREATE TABLE STUDENT(
   Name varchar(35), 
   Age INT, 
   Score INT
);

Following query creates a trigger, this will set the score value 0 if you enter a value that is less than 0 as score.

DELIMITER //
CREATE TRIGGER sample_trigger BEFORE INSERT ON STUDENT FOR EACH ROW
BEGIN
IF NEW.Score < 0 THEN SET NEW.Score = 0;
END IF;
END //
DELIMITER ;

Verification

If you try to insert records in the student table and if you use a value that is less than 0 as age it will be automatically set to 0.

INSERT INTO STUDENT VALUES
('Jeevan', 22, 8),
('Raghav', 26, -3),
('Pooja', 21, -9),
('Devi', 30, 9);

The STUDENT table created will have the following records −

Name Age Score
Jeevan 22 8
Raghav 26 0
Pooja 21 0
Devi 30 9

As we can see, there are no negative values inserted in the table as they are all replaced with zeroes.

Creating Trigger Using a Client Program

We can also Create a trigger using a client program.

Syntax

PHP NodeJS Java Python

To Create a Trigger through a PHP program, we need to execute the CREATE statement using the mysqli function named query() as follows −

$sql = "CREATE TRIGGER testTrigger AFTER UPDATE ON Student FOR EACH ROW INSERT INTO Student SET action = 'update', Name = OLD.Name, age = OLD.age, score = OLD.score";
$mysqli->query($sql);

To Create a Trigger through a JavaScript program, we need to execute the CREATE statement using the query() function of mysql2 library as follows −

sql = "CREATE TRIGGER testTrigger AFTER UPDATE ON Student FOR EACH ROW INSERT INTO Student SET action = 'update', Name = OLD.Name, age = OLD.age, score = OLD.score"
con.query(sql);

To Create a Trigger through a Java program, we need to execute the CREATE statement using the JDBC function execute() as follows −

String sql = "CREATE TRIGGER testTrigger AFTER UPDATE ON Student FOR EACH ROW INSERT INTO Student SET action = 'update', Name = OLD.Name, age = OLD.age, score = OLD.score";
statement.execute(sql);

To Create a Trigger through a python program, we need to execute the CREATE statement using the execute() function of the MySQL Connector/Python as follows −

create_trigger_query = 'CREATE TRIGGER sample_trigger
BEFORE INSERT ON students
FOR EACH ROW
BEGIN
   IF NEW.Score < 0 THEN
      SET NEW.Score = 0;
   END IF;
END'
cursorObj.execute(create_trigger_query)

Example

Following are the programs −

PHP NodeJS Java Python 
$dbhost = 'localhost';
$dbuser = 'root';
$dbpass = 'password';
$db = 'TUTORIALS';
$mysqli = new mysqli($dbhost, $dbuser, $dbpass, $db);
if($mysqli->connect_errno ) {
   printf("Connect failed: %s
", $mysqli->connect_error);
   exit();
}
//printf('Connected successfully.
');
$sql = "CREATE TRIGGER testTrigger AFTER UPDATE ON Student FOR EACH ROW INSERT INTO Student SET action = 'update', Name = OLD.Name, age = OLD.age, score = OLD.score";
if($mysqli->query($sql)){
   printf("Trigger created successfully...!");
}
if($mysqli->error){
   printf("Failed..!" , $mysqli->error);
}
$mysqli->close();

Output

The output obtained is as follows −

Trigger created successfully...!

var mysql = require('mysql2');
var con = mysql.createConnection({
   host:"localhost",
   user:"root",
   password:"password"
});

//Connecting to MySQL
con.connect(function(err) {
   if (err) throw err;
   //console.log("Connected successfully...!");
   //console.log("--------------------------");
   sql = "USE TUTORIALS";
   con.query(sql);
   sql = "CREATE TRIGGER testTrigger AFTER UPDATE ON Student FOR EACH ROW INSERT INTO Student SET action = 'update', Name = OLD.Name, age = OLD.age, score = OLD.score"
   con.query(sql);
   console.log("Trigger created successfully....!");
   sql = "SHOW TRIGGERS";
   con.query(sql, function(err, result){
      if (err) throw err;
      console.log(result);
   });
});

Output

The output produced is as follows −

Trigger created successfully....!
[
  {
    Trigger: 'testTrigger',
    Event: 'UPDATE',
    Table: 'student',
    Statement: "INSERT INTO Student SET action = 'update', Name = OLD.Name, age = OLD.age, score = OLD.score",
    Timing: 'AFTER',
    Created: 2023-08-01T05:21:18.540Z,
    sql_mode: 'IGNORE_SPACE,STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION',
    Definer: 'root@localhost',
    character_set_client: 'utf8mb4',
    collation_connection: 'utf8mb4_unicode_ci',
    'Database Collation': 'utf8mb4_0900_ai_ci'
  }
] 

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;
public class CreateTrigger {
   public static void main(String[] args) {
      String url = "jdbc:mysql://localhost:3306/TUTORIALS";
      String user = "root";
      String password = "password";
      ResultSet rs;
      try {
         Class.forName("com.mysql.cj.jdbc.Driver");
         Connection con = DriverManager.getConnection(url, user, password);
         Statement st = con.createStatement();
         //System.out.println("Database connected successfully...!");
         String sql = "CREATE TRIGGER testTrigger AFTER UPDATE ON Student FOR EACH ROW INSERT INTO Student SET action = 'update', Name = OLD.Name, age = OLD.age, score = OLD.score";
         st.execute(sql);
         System.out.print("Trigger created successfully....!");
            
      }catch(Exception e) {
         e.printStackTrace();
      }
   }
}

Output

The output obtained is as shown below −

Trigger created successfully....!

import mysql.connector
#Establishing the connection
connection = mysql.connector.connect(
   host='localhost',
   user='root',
   password='password',
   database='tut'
)
table_name = 'Student'
trigger_name = 'sample_trigger'
# Creating a cursor object
cursorObj = connection.cursor()
# Create table
create_table_query = '''
CREATE TABLE Student(
   Name VARCHAR(35),
   age INT,
   Score INT
)
'''
cursorObj.execute(create_table_query)
print("The table is created successfully!")
# Creating a trigger
create_trigger_query = f'''CREATE TRIGGER {trigger_name}
BEFORE INSERT ON {table_name}
FOR EACH ROW
BEGIN
   IF NEW.Score < 0 THEN
      SET NEW.Score = 0;
   END IF;
END'''
cursorObj.execute(create_trigger_query)
print(f"Trigger '{trigger_name}' is created successfully.")
# Commit the changes and close the cursor and connection
connection.commit()
cursorObj.close()
connection.close()

Output

Following is the output of the above code −

The table is created successfully!
Trigger 'sample_trigger' is created successfully.
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