Working with Triggers in a MySQL Database - A Tutorial

A trigger is a pre-defined SQL command that is automatically executed when specific actions occur in the database. It can be fired either before or after an INSERT, UPDATE, or DELETE event.

Triggers are mainly used to maintain software logic in the MySQL server, and they have several benefits:

• Triggers help keep global operations centralized in one location.

• They reduce client-side code and help minimize the round-trips made to the database server.

• They help make applications more scalable across different platforms.

Some common use-cases of triggers include audit logging, pre-computing database values (e.g. cumulative sums), and enforcing complex data integrity and validation rules.

In this guide, you will learn:

• How the syntax for a trigger is structured.

• How to create triggers that are executed before other database events occur.

• How to create triggers that are executed after other database events occur.

• How to delete triggers.

Before You Begin

Make sure you have the following:

1. A configured Linode server. You can learn how to create and setup a Linode server by reading our Getting Started with Linode guide.

2. A MySQL server and client installed on the Linode server. Installation guides for MySQL are available for different distributions in our MySQL section.

Prepare the Database

To better understand how triggers work, we will create a sample database and add sample data into it. Later, we will create different triggers on the database as a proof of concept exercise.

1. First, log in to your MySQL Server:

   mysql -u root -p
   

   Then, enter the root password of your MySQL server and hit Enter to proceed.

2. Next, you will see a MySQL prompt similar to the one shown below:

   mysql >

3. Create a test_database by running the command below:

   CREATE DATABASE test_database;
   

   Output:

   Query OK, 1 row affected (0.02 sec)

4. Switch to the database:

   USE test_database;
   

   Output:

   Database changed

5. Once the database is selected, we will create some tables that we will use for demonstrating triggers. We will begin by creating the stores table. This table will hold information about two sample stores/offices where our hypothetical business operates from:

   CREATE TABLE stores
   (
   store_id BIGINT PRIMARY KEY AUTO_INCREMENT,
   store_name VARCHAR(50)
   ) ENGINE=InnoDB;
   

   Output:

   Query OK, 0 rows affected (0.07 sec)

6. Next, add two records to the stores table by running the commands below:

   INSERT INTO stores (store_name) VALUES ('Philadelphia');
   INSERT INTO stores (store_name) VALUES ('Galloway');
   

   After each command, you will get the below output:

   Query OK, 1 row affected (0.08 sec)
   ...

7. Confirm the records by running the command below:

   SELECT * FROM stores;
   

   Output:

   +----------+--------------+
   | store_id | store_name   |
   +----------+--------------+
   |        1 | Philadelphia |
   |        2 | Galloway     |
   +----------+--------------+
   2 rows in set (0.01 sec)

8. Next, create the products table. The table will hold different products being offered in the store:

   CREATE TABLE products
   (
   product_id BIGINT PRIMARY KEY AUTO_INCREMENT,
   product_name VARCHAR(40),
   cost_price DOUBLE,
   retail_price DOUBLE,
   availability VARCHAR(5)
   ) ENGINE=InnoDB;
   

   Output:

   Query OK, 0 rows affected (0.13 sec)

   • Each product will be uniquely identified by a product_id.

   • A product_name field will specify the names of the items.

   • The cost_price and retail_price fields will determine the buying and selling price respectively.

   • An availability column will define the product availability in the different stores. If the product is only available in our local store (Philadelphia), we will denote it with a LOCAL value. Else, we will use the value of ALL to signify a product that is available in both stores (Philadelphia and Galloway).

9. Add sample data to the products table:

   INSERT INTO products (product_name, cost_price, retail_price, availability) VALUES ('WIRELESS MOUSE', '18.23', '30.25','ALL');
   
   INSERT INTO products (product_name, cost_price, retail_price, availability) VALUES ('8 MP CAMERA', '60.40', '85.40','ALL');
   
   INSERT INTO products (product_name, cost_price, retail_price, availability) VALUES ('SMART WATCH', '189.60', '225.30','LOCAL');
   

   You will get the output shown below after each insert command:

   Query OK, 1 row affected (0.02 sec)
   ...

10. Confirm if the products were inserted by running the command below:

    SELECT * FROM products;
    

    Output:

    +------------+----------------+------------+--------------+--------------+
    | product_id | product_name   | cost_price | retail_price | availability |
    +------------+----------------+------------+--------------+--------------+
    |          1 | WIRELESS MOUSE |      18.23 |        30.25 | ALL          |
    |          2 | 8 MP CAMERA    |       60.4 |         85.4 | ALL          |
    |          3 | SMART WATCH    |      189.6 |        225.3 | LOCAL        |
    +------------+----------------+------------+--------------+--------------+
    3 rows in set (0.00 sec)

11. Next, the products' availability will be mapped to another table named products_to_stores. This table will just reference the product_id from the products table and the store_id from the stores table where the item is available.

    Create the products_to_stores table by running the code below:

    CREATE TABLE products_to_stores
    (
    ref_id BIGINT PRIMARY KEY AUTO_INCREMENT,
    product_id BIGINT,
    store_id BIGINT
    ) ENGINE=InnoDB;
    

    Output:

    Query OK, 0 rows affected (0.14 sec)

12. Next, we will create an archived_products table. The table will hold information about deleted products for future reference:

    CREATE TABLE archived_products
    (
    product_id BIGINT PRIMARY KEY ,
    product_name VARCHAR(40),
    cost_price DOUBLE,
    retail_price DOUBLE,
    availability VARCHAR(5)
    ) ENGINE=InnoDB;
    

    Output:

    Query OK, 0 rows affected (0.14 sec)

13. Lastly, we will create a products_price_history table for tracking the different prices of each product over time:

    CREATE TABLE products_price_history
    (
    product_id BIGINT PRIMARY KEY AUTO_INCREMENT,
    price_date DATETIME,
    retail_price DOUBLE
    ) ENGINE=InnoDB;
    

    Output:

    Query OK, 0 rows affected (0.14 sec)

Once our database structure is in place, we can now go ahead and learn the basic syntax of a MySQL database trigger in order to create our first sample.

Trigger Syntax

As indicated earlier, triggers are fired automatically either before or after an SQL command is run in the database. The basic syntax for creating triggers is as follows:

CREATE TRIGGER TRIGGER_NAME

TRIGGER_TIME TRIGGER_EVENT

ON TABLE_NAME FOR EACH ROW

[TRIGGER BODY];

• TRIGGER_NAME: Each trigger must have a unique name and you should define it here.

• TRIGGER_TIME: Either BEFORE or AFTER.

• TRIGGER_EVENT: You need to specify the database event that will invoke the trigger: INSERT, UPDATE, or DELETE.

• TRIGGER BODY: This specifies the actual SQL command (or commands) that you want to be run by your trigger.

If a trigger body has more than one SQL statement, you must enclose it within a BEGIN...END block. As well, you will need to temporarily change the DELIMITER that signals the end of the trigger body to a new value. This ensures that the statements within the body are not prematurely interpreted by your MySQL client. An example of this looks like the following:

DELIMITER &&

CREATE TRIGGER TRIGGER_NAME

TRIGGER_TIME TRIGGER_EVENT

ON TABLE_NAME FOR EACH ROW

BEGIN

[TRIGGER BODY]

END &&

DELIMITER ;

Note

The last line of this example changes the DELIMITER back to the default ; value.

Creating Before Event Triggers

In this section, we will look into the different types of triggers that are fired before a database operation. These include the BEFORE INSERT, BEFORE UPDATE, and BEFORE DELETE triggers.

Creating a Before Insert Trigger

We will create our first BEFORE INSERT trigger. The trigger will make sure that the retail price of a product is greater than the cost price whenever items are inserted into the products table. Otherwise, the database user will get an error.

1. While still on the mysql > prompt, enter the command below:

   DELIMITER $$
   
   CREATE TRIGGER price_validator
   
   BEFORE INSERT
   
   ON products FOR EACH ROW
   
   IF NEW.cost_price>=NEW.retail_price
   
   THEN
   
   SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Retail price must be greater than cost price.';
   
   END IF $$
   
   DELIMITER ;
   

   • The above code defines the trigger name (price_validator), time (BEFORE), event (INSERT), and the table (products) to be affected.

   • Our trigger uses the NEW keyword to check the cost_price and retail_price before a record is inserted to the products table, using the IF...THEN...END IF statement.

   • If the cost_price is greater or equal to the retail price, our triggers tells MySQL to throw a custom exception instructing the user to rectify the error.

2. To test the trigger above, try inserting a product that violates the validation rule:

   INSERT INTO products (product_name, cost_price, retail_price, availability) VALUES ('GAMING MOUSE PAD', '145.00', '144.00','LOCAL');
   

   Output:

   ERROR 1644 (45000): Retail price must be greater than cost price.

   The above insert commands should fail because the retail_price (144.00) is not greater than the cost_price (145.00).

Creating a Before Update Trigger

Next, we will create a BEFORE UPDATE trigger. This trigger will prevent database users from editing a product name once a product has been inserted into the database. If you have multiple users working in the database, a BEFORE UPDATE trigger may be used to make values read-only, and this can prevent malicious or careless users from modifying records unnecessarily.

1. Create a new product_name_validator trigger with the command below:

   DELIMITER $$
   
   CREATE TRIGGER product_name_validator
   
   BEFORE UPDATE
   
   ON products FOR EACH ROW
   
   IF NEW.product_name<>OLD.product_name
   
   THEN
   
   SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Product name is read-only and it can not be changed.';
   
   END IF $$
   
   DELIMITER ;
   

   This trigger compares the values of the new product_name (NEW.product_name) and the old name already in the database (OLD.product_name). If there is a mismatch, an exception is thrown.

2. To invoke the product_name_validator trigger, we can attempt to update the name of the product with the ID 1:

   UPDATE products SET product_name='WIRELESS BLUETOOTH MOUSE' WHERE product_id='1';
   

   Output:

   ERROR 1644 (45000): Product name is read-only and it can not be changed.

Defining a Before Delete Trigger

In this section, you will see how you can define a BEFORE DELETE trigger to prevent users from deleting specific records from a table.

1. To create the prevent_delete trigger, run the command below:

   DELIMITER $$
   
   CREATE TRIGGER prevent_delete
   
   BEFORE DELETE
   
   ON products FOR EACH ROW
   
   IF OLD.availability='ALL'
   
   THEN
   
   SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'The product can not be deleted because it is available in ALL stores.';
   
   END IF $$
   
   DELIMITER ;
   

   This trigger will prevent products marked with a value of ALL in the availability column from being deleted.

2. Next, try to delete the first product from the products table and see if the trigger will be invoked:

   DELETE FROM products WHERE product_id='1';
   

   Output:

   ERROR 1644 (45000): The product can not be deleted because it is available in ALL stores.

We have looked at the different triggers that are invoked before a database operation. Next, we will look into the other types of triggers that are fired after database events.

Creating After Event Triggers

In a production environment, you may want some triggers to be automatically executed after a database event occurs (for example, inserting records into different tables). The examples below demonstrate how these kinds of triggers can be used in our sample database.

Creating an After Insert Trigger

This example creates a trigger named product_availability that inserts mapping records into the products_to_stores table. This trigger is used to enforce business logic; in particular, it helps define the product availability for the different stores.

1. Run the code below to create the product_availability trigger. Since we have multiple lines of code in the trigger body, we will use a BEGIN...END block:

   DELIMITER $$
   
   CREATE TRIGGER product_availability
   
   AFTER INSERT
   
   ON products FOR EACH ROW
   
   BEGIN
   
   IF NEW.availability='LOCAL' then
   
   INSERT INTO products_to_stores (product_id, store_id) VALUES (NEW.product_id, '1');
   
   ELSE
   
   INSERT INTO products_to_stores (product_id, store_id) VALUES (NEW.product_id, '1');
   
   INSERT INTO products_to_stores (product_id, store_id) VALUES (NEW.product_id, '2');
   
   END IF;
   
   END $$
   
   DELIMITER ;
   

   • When an item is being inserted into the products table, the trigger will check the availability field.

   • If it is marked with the LOCAL value, the product will be made available in one store only.

   • Any other value will instruct the trigger to make the product available to the two stores that we created earlier.

2. To see the product_availability trigger in action, insert the two records to the products table:

   INSERT INTO products (product_name, cost_price, retail_price, availability) VALUES ('BLUETOOTH KEYBOARD', '17.60', '23.30','LOCAL');
   INSERT INTO products (product_name, cost_price, retail_price, availability) VALUES ('DVB-T2 RECEIVE', '49.80', '53.40','ALL');
   

3. Then, query the products_to_stores table:

   SELECT * FROM products_to_stores;
   

   You should see an output similar to the one shown below:

   +--------+------------+----------+
   | ref_id | product_id | store_id |
   +--------+------------+----------+
   |      1 |          4 |        1 |
   |      2 |          5 |        1 |
   |      3 |          5 |        2 |
   +--------+------------+----------+
   3 rows in set (0.00 sec)

Defining an After Update Trigger

A trigger can also be fired after an UPDATE event. We will see how we can leverage this type of trigger to keep track of price changes in our store over time.

1. Create a product_history_updater trigger by running the command below:

   CREATE TRIGGER product_history_updater
   
   AFTER UPDATE
   
   ON products FOR EACH ROW
   
   INSERT INTO products_price_history (product_id, price_date, retail_price) VALUES (OLD.product_id, NOW(), NEW.retail_price);
   

   This trigger records changes to a product’s retail_price in the products_price_history table.

   Note

   Unlike previous examples, this trigger only has one statement in the trigger’s body, so we do not need to change the DELIMITER.

2. Then, try updating the price of the first product by running the command below:

   UPDATE products SET retail_price='36.75' WHERE product_id='1';
   

3. Next, query the products_price_history table to see if the price change was logged:

   SELECT * FROM products_price_history;
   

   If the trigger worked as expected, you should get the below output:

   +------------+---------------------+--------------+
   | product_id | price_date          | retail_price |
   +------------+---------------------+--------------+
   |          1 | 2020-01-28 11:46:21 |        36.75 |
   +------------+---------------------+--------------+
   1 row in set (0.00 sec)

Creating an After Delete Trigger

In some cases, you might want to log delete operations after a specific action has occurred in the database. You can achieve this by using the AFTER DELETE trigger.

1. Create a new the product_archiver trigger with the command below:

   CREATE TRIGGER product_archiver
   
   AFTER DELETE
   
   ON products FOR EACH ROW
   
   INSERT INTO archived_products (product_id, product_name, cost_price, retail_price, availability) VALUES (OLD.product_id, OLD.product_name, OLD.cost_price, OLD.retail_price, OLD.availability);
   

   This trigger archives deleted products in a separate table named archived_products. When an item is deleted from the main products table, our trigger will automatically log it to the archived_products table for future reference.

2. Next, delete a product from the products table and see if the trigger will be invoked:

   DELETE FROM products WHERE product_id='3';
   

3. Now, if you check the archived_products table, you should see one record:

   SELECT * FROM archived_products;
   

   Output:

   +------------+--------------+------------+--------------+--------------+
   | product_id | product_name | cost_price | retail_price | availability |
   +------------+--------------+------------+--------------+--------------+
   |          3 | SMART WATCH  |      189.6 |        225.3 | LOCAL        |
   +------------+--------------+------------+--------------+--------------+
   1 row in set (0.00 sec)

Deleting a Trigger

You have seen the different types of triggers and how they can be used in a production environment. Sometimes, you may want to remove a trigger from the database.

You can delete a trigger if you don’t want to use it anymore using the syntax below:

DROP TRIGGER IF EXISTS TRIGGER_NAME;

Note

The IF EXISTS keyword is an optional parameters that only deletes a trigger if it exists.

For example, to delete the product_archiving trigger that we defined above, use the below command:

DROP TRIGGER IF EXISTS product_archiver;

Output:

Query OK, 0 rows affected (0.00 sec)

Caution

Be cautious when deleting tables associated with triggers. Once a table is dropped from the MySQL database, the related triggers are also automatically deleted.

More Information

You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.

• MySQL Trigger Syntax and Examples