Trigger in DBMS

Last Updated on December 27, 2023 by Ankit Kochar

In the realm of Database Management Systems (DBMS), triggers stand as powerful mechanisms that automatically execute a set of actions in response to certain events occurring in the database. They serve as indispensable tools for maintaining data integrity, enforcing constraints, and automating tasks within the database environment. Triggers essentially act as guardians, intercepting data operations and executing predefined actions to ensure the database remains consistent and secure.

Understanding triggers is pivotal for database developers, administrators, and anyone involved in database management. This article delves into the essence of triggers, their types, functionalities, and their significance within a DBMS environment. By exploring the core concepts and practical applications, readers will gain a comprehensive understanding of how triggers operate and their pivotal role in ensuring the reliability and consistency of databases.

What is the Trigger in DBMS?

Trigger in DBMS is a special type of stored procedure that is automatically executed in response to certain database events such as an INSERT, UPDATE, or DELETE operation. Triggers can be used to perform actions such as data validation, enforcing business rules, or logging. They can be defined to execute before or after the triggering event and can be defined to execute for every row or once for each statement. Triggers are a powerful feature of dbms that allow developers to define automatic actions based on database events.

Types of Trigger in DBMS on Basis of their Execution

Triggers in Dbms can be classified into two types based on when they are executed:

• BEFORE Trigger: A BEFORE trigger is executed before the triggering event (such as an insert, update, or delete) is executed on the table. This type of trigger is often used to modify the data before it is inserted, updated, or deleted.
• AFTER Trigger: An AFTER trigger is executed after the triggering event has been executed on the table. This type of trigger is often used to perform actions that depend on the changes made to the table due to the triggering event.

Types of Trigger in DBMS on Basis of Event or Action Triggering Them

Triggers can also be classified based on the event or action that triggers them. The most commonly used types of triggers based on this classification are:

• INSERT Trigger: This type of trigger occurs when new data is inserted into a table.
• UPDATE Trigger: This type of trigger occurs when existing data in a table is updated.
• DELETE Trigger: This type of trigger occurs when data is deleted from a table.
• INSTEAD OF Trigger: This type of trigger is used to replace the regular insert, update, or delete operation on a view with a custom operation. It is fired instead of the regular operation.

Creating a Trigger in DBMS:

Let us create a trigger in DBMS:

CREATE TRIGGER trigger_name
[BEFORE|AFTER] [INSERT|UPDATE|DELETE] ON table_name
[FOR EACH ROW]
BEGIN
  -- code to be executed when the trigger is activated
END;

Let’s break down each part of this code:

• CREATE TRIGGER: This is the SQL command used to create a trigger.
• trigger_name: This is the name of the trigger. You can choose any name you like, as long as it follows the naming conventions of your DBMS.
• [BEFORE|AFTER]: This specifies whether the trigger should be activated before or after the triggering event.
• [INSERT|UPDATE|DELETE]: This specifies the event that will trigger the execution of the trigger. You can choose one or more of these options depending on the specific trigger you want to create.
• ON table_name: This specifies the table that the trigger will be associated with.
• [FOR EACH ROW]: This specifies that the trigger will be executed once for each row that is affected by the triggering event.
• BEGIN and END: This is where you will put the code that should be executed when the trigger is activated. The code inside the BEGIN and END blocks can be any SQL statements that you want to execute when the trigger is activated. The exact syntax and functionality of triggers can vary depending on the specific DBMS you are using.

Example of Trigger in DBMS:

Suppose we have a table named "employees" with columns "id", "name", "age", and "salary". We want to ensure that whenever a new employee is added to the table, the age of the employee must be greater than or equal to 18. We can create a BEFORE INSERT trigger to enforce this rule. Here’s how the trigger might look:

Code of Trigger in DBMS:

CREATE TRIGGER age_check
BEFORE INSERT ON employees
FOR EACH ROW
BEGIN
    IF NEW.age < 18 THEN
        SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Employees must be at least 18 years old';
    END IF;
END;

Explanation of Trigger in DBMS:
This trigger will be fired before any new row is inserted into the "employees" table. It checks the age of the new employee being inserted, and if it is less than 18, it raises an error message using the SIGNAL statement, which prevents the insertion of the new row. This trigger ensures that the age constraint is enforced automatically every time a new employee is added to the table, reducing the chances of data inconsistency and improving data quality.

Advantages of Trigger in DBMS:

Some of the advantages of trigger in DBMS:

• They can help ensure that data remains consistent and accurate by enforcing rules and constraints on the data.
• These can simplify the implementation of complex business logic that requires multiple steps or queries to be executed.
• It can be used to log changes to the database, providing an audit trail that can be used for compliance, debugging, or analysis purposes.
• Triggers can automate tasks that would otherwise require manual intervention, improving the efficiency and accuracy of database operations.

Disadvantages of Trigger in DBMS:

Some of the disadvantages of trigger in DBMS:

• Triggers in DBMS can impact database performance, especially when they involve complex operations or are triggered frequently.
• They can introduce hidden logic into the database, which can make it more difficult to understand and maintain.
• These can make it harder to debug issues in the database because they are executed automatically and can be difficult to trace.
• It can introduce security vulnerabilities if they are not properly designed or tested, potentially allowing unauthorized access or modification of data.

Conclusion:
Triggers in a DBMS play an integral role in automating tasks, maintaining data integrity, and enforcing business rules within a database environment. By reacting to specific events like INSERT, UPDATE, DELETE, triggers execute predefined actions that help ensure data consistency and enforce complex constraints that might otherwise be difficult to manage manually.

Their ability to encapsulate business logic within the database itself improves efficiency, reduces redundancy, and enhances data security. Nevertheless, the creation and management of triggers require careful planning to avoid unintended consequences such as performance issues or unexpected behavior within the database.

As a fundamental feature in database systems, understanding triggers empowers database administrators and developers to harness their potential effectively, ensuring the reliability, security, and integrity of the database.

Frequently Asked Questions(FAQS) Related to trigger in DBMS:

Here are some FAQs about Trigger in DBMS.

1. What exactly is a trigger in a database?
A trigger in a database is a set of instructions that automatically executes in response to specific events occurring in the database, such as INSERT, UPDATE, or DELETE operations on a table. Triggers are defined to enforce constraints, maintain data integrity, or automate tasks within the database.

2. What are the types of triggers in DBMS?
In general, triggers in DBMS are categorized as 'Row-level triggers' and 'Statement-level triggers.' Row-level triggers activate for each affected row due to a triggering event, while statement-level triggers execute once for each triggering event, regardless of the number of rows affected.

3. How do triggers differ from stored procedures?
Triggers and stored procedures serve different purposes. Triggers are event-driven and automatically execute in response to specific database events, while stored procedures are pre-defined sets of SQL statements that are explicitly called by users or applications.

4. What are some common use cases for triggers in a DBMS?
Triggers are commonly used to enforce data integrity by implementing constraints, auditing changes made to tables, automatically updating related tables when certain actions occur, and enforcing complex business rules within the database.

5. Are there any drawbacks to using triggers?
While triggers offer significant benefits, they can also introduce complexity to the database, impacting performance and potentially causing unexpected behavior if not implemented carefully. Overuse of triggers or complex logic within triggers can make database maintenance and troubleshooting more challenging.