What is a Trigger?

 

A trigger is a series of actions that is activated when a defined event occurs for a specific table. This event can either be an INSERT, UPDATE or DELETE. Triggers can be invoked before or after the event.

 

To understand how triggers work, let’s look at an example.

 

We’ll use the employees table to demonstrate.

 

Suppose one of the employees has just resigned from the company and we want to delete this employee from the employees table. However, before we do that, we would like to transfer the data into another table called ex_employees as a form of back up. We can do this using a trigger.

 

Let’s first create an ex_employees table using the code below:

 

CREATE TABLE ex_employees (

em_id INT PRIMARY KEY,

em_name VARCHAR(255) NOT NULL,

gender CHAR(1) NOT NULL,

date_left TIMESTAMP DEFAULT NOW()

);