A **trigger** is a stored procedure - which is a set of SQL statements saved under a name, just like a function, so that it can be reused - that is executed automatically when a certain event occurs in a database.

For instance, you may program a trigger to execute when data is inserted in a table.
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## Benefits
- Calculating derived columns automatically
- Ensuring referential integrity
- Logging events 
- Auditing
- Replicating tables synchronously instead of the default asynchronous replication
- Authenticating users
- Preventing invalid transactions



# Types of Triggers
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## 1. Event
- **DML Trigger:** It fires when a `DML` (Database Manipulation Language) event is specified (`INSERT`/`UPDATE`/`DELETE`)
- **DDL Trigger:** It fires when a `DDL` (Database Definition Language) event is specified (`CREATE`/`ALTER`)
- **DATABASE Trigger:** It fires when a database event is specified (`LOGON`/`LOGOFF`/`STARTUP`/`SHUTDOWN`)
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## 2. Timing
- **BEFORE Trigger:** It fires before the specified event has occurred.
- **AFTER Trigger:** It fires after a specified event has occurred.
- **INSTEAD OF Trigger:** It lets you skip a statement and execute a different statement present in the trigger body instead. 
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## 3. Level
- **STATEMENT level Trigger:** It fires one time for a specified event statement.
- **ROW level Trigger:** It fires for each record that was affected in a specified event. (only for `DML`)


# SQL Implementation

A *trigger* is implemented in the given code which will be fired before anything is inserted in the `people` table. This trigger makes sure that a person does not have a negative `age` and sets the age to zero. 

CREATE TABLE people (
  age INT, 
  name varchar(150)
);

delimiter //
CREATE TRIGGER agecheck BEFORE INSERT ON people FOR EACH ROW IF NEW.age < 0 THEN SET NEW.age = 0; END 
IF; //

INSERT INTO people VALUES (-20, 'Sid'), (30, 'Josh');
select * from people;

What are triggers in SQL?

A trigger is an automatic stored procedure in SQL that executes certain events, ensuring actions like data integrity, logging, and user authentication.