All about Triggers in Sql Server & Oracle

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What is a Trigger in SQL Server?

A Trigger is a database object that is attached to a table. In many aspects Trigger's are quite similar to a Stored Procedure. That's why, triggers are sometimes referred to as a 'special kind of stored procedure'. The main difference between a trigger and a stored procedure is that the former is attached to a table and is only fired when an INSERT, UPDATE or DELETE occurs or you can say a trigger is called automatically when a data modification event is made against a table whereas a stored procedure must be called explicitly.

Features Of Trigger:-

1. A batch of SQL code for an insert, update or delete command is executed.

2. Business rules can be enforced on modification of data.

Advantages of using SQL triggers:-

1. SQL triggers provide an alternative way to check the integrity of data.

2. SQL triggers can catch errors in business logic in the database layer.

3. SQL triggers provide an alternative way to run scheduled tasks. By using SQL triggers, you don’t have to wait to run the scheduled tasks because the triggers are invoked automatically before or after a change is made to the data in tables.

4. SQL triggers are very useful to audit the changes of data in tables.

Disadvantages of using SQL triggers:-

1. Does not accept arguments or parameters.

2. Cannot perform commit or rollback.

3. Can cause table errors if poorly written.

4. SQL triggers only can provide an extended validation and they cannot replace all the validations. Some simple validations have to be done in the application layer. For example, you can validate user’s inputs in the client side by using JavaScript or in the server side using server side scripting languages such as JSP, PHP, ASP.NET, Perl, etc.

5. SQL triggers are invoked and executed invisibly from client-applications therefore it is difficult to figure out what happen in the database layer.

6. SQL triggers may increase the overhead of the database server.

Types of Triggers:-

In Sql Server we can create four types of triggers:- and Logon triggers.

1). Data Definition Language (DDL) triggers: In SQL Server we can create triggers on DDL statements like CREATE, ALTER, and DROP and certain system defined stored procedures that perform DDL-like operations.

e.g. If you are going to execute the CREATE LOGIN statement or the sp_addlogin stored procedure to create login user, then both these can execute/fire a DDL trigger that you can create on CREATE_LOGIN event of Sql Server.

We can use only FOR/AFTER clause in DDL triggers not INSTEAD OF clause means we can make only After Trigger on DDL statements.

DDL trigger can be used to observe and control actions performed on the server, and to audit these operations. DDL triggers can be used to manage administrator tasks such as auditing and regulating database operations.

2). Data Manipulation Language (DML) triggers: In SQL Server we can create triggers on DML statements like INSERT, UPDATE, and DELETE and stored procedures that perform DML-like operations. DML Triggers are of two types:

2.1). After Trigger (using FOR/AFTER CLAUSE): This trigger fires after SQL Server finish the execution of the action successfully that fired it.

e.g: If you insert record/row in a table then the trigger related/associated with the insert event on this table will fire only after the row passes all the constraints, like as primary key constraint, and some rules. If the record/row insertion fails, SQL Server will not fire the After Trigger.

2.2). Instead of Trigger (using INSTEAD OF CLAUSE) : This type of trigger is fired before SQL Server starts the execution of the action that fired it. This is differ from the AFTER trigger, which fires after the action that caused it to fire. We can have an INSTEAD OF insert/update/delete trigger on a table that successfully executed but does not include the actual insert/update/delete to the table.

e.g: If you insert record/row in a table then the trigger related/associated with the insert event on this table will fire before the row passes all the constraints, such as primary key constraint and some rules. If the record/row insertion fails, SQL Server will fire the Instead of Trigger.

3). CLR Triggers: These are special type of triggers that based on the Common Language Runtime(CLR) in .net framework. CLR integration of triggers has been introduced with SQL Server 2008 and allows for triggers to be coded in one of .NET languages like C#, Visual Basic and F#.

4). Logon Triggers: These are special type of trigger that fire when LOGON event of Sql Server is raised. This event is raised when a user session is being established with Sql Server that is made after the authentication phase finishes, but before the user session is actually established. Hence, all messages that we define in the trigger such as error messages, will be redirected to the SQL Server error log. Logon triggers do not fire if authentication fails. We can use these triggers to audit and control server sessions, such as to track login activity or limit the number of sessions for a specific login.

Different modes of firing triggers?

After Trigger: An AFTER trigger fires after SQL Server completes all actions successfully

Instead of Triggers: An INSTEAD OF trigger causes SQL Server to execute the code in the trigger instead of the operation that caused the trigger to fire.

Differences between triggers and non-trigger stored procedures are (amongst others):

1). A non-trigger stored procedure is like a program that has to be invoked explicitly either from code or from a scheduler or from a batch job, etc. to do its work, whereas a trigger is a special type of stored procedure that fires as a response of an event rather than be directly executed by the user. The event may be a change of data in a data column for example.

2). Triggers have types e.g DDL Triggers and DML Triggers (of types: INSTEAD OF, For, and AFTER)

3). Non-Trigger Stored procedures can reference any type of object, however, to reference a view, you must use INSTEAD OF triggers.

4). In SQLServer, you can have any number on non-trigger stored procedures but only 1 INSTEAD OF trigger per table.

5). We can call a stored procedure from front end (.asp files, .aspx files, .ascx files etc.) but we can't call a trigger from these files.

6). Stored procedure can take the input parameters, but we can't pass the parameters as an input to a trigger.

Note:

* DML trigger can be composed by any T-SQL statements, except CREATE DATABASE, ALTER DATABASE, DROP DATABASE, LOAD DATABASE, LOAD LOG, RECONFIGURE, RESTORE DATABASE, and RESTORE LOG statements.

* You cannot create triggers against system tables or dynamic management views.

* Also, the TRUNCATE TABLE statement does not fire a trigger because this operation does not log individual row deletions.

* If you use the DATABASE option, the scope of your DDL trigger will be the current database. If you use the ALL SERVER option, the scope of your DDL triggers to the current server.

PL/SQL Syntax to write a trigger:

CREATE [OR REPLACE ] TRIGGER trigger_name

{BEFORE | AFTER | INSTEAD OF }

{INSERT [OR] | UPDATE [OR] | DELETE}

[OF col_name]

ON table_name

[REFERENCING OLD AS o NEW AS n]

[FOR EACH ROW]

WHEN (trigger_condition)

DECLARE

Declaration-statements

BEGIN

trigger_body

EXCEPTION

Exception-handling-statements

END;

# CREATE [OR REPLACE] TRIGGER trigger_name: Creates or replaces an existing trigger with the trigger_name.

# BEFORE specifies the trigger fires before the triggering event is performed.

# AFTER specifies the trigger fires after the triggering event is performed.

# INSTEAD OF specifies the trigger fires instead of performing the triggering event.The INSTEAD OF clause is used for creating trigger on a view.

# {INSERT [OR] | UPDATE [OR] | DELETE}: This specifies the DML operation.

# [OF col_name]: This specifies the column name that would be updated.

# ON table_name: This specifies the name of the table associated with the trigger.

# REFERENCING OLD AS o NEW AS n: This allows you to refer new and old values for various DML statements, like INSERT, UPDATE, and DELETE.

# FOR EACH ROW: This specifies a row level trigger, i.e., the trigger would be executed for each row being affected. Otherwise the trigger will execute just once when the SQL statement is executed, which is called a table level trigger.

# WHEN (trigger_condition): This provides a condition for rows for which the trigger would fire. This clause is valid only for row level triggers.

# trigger_body contains the SQL and PL/SQL statements that perform the trigger's task.

NOTE:

There are two types of triggers based on the which level it is triggered.

1) Row level trigger: An event is triggered for each row upated, inserted or deleted.

2) Statement level trigger: An event is triggered for each sql statement executed.

e.g Create a trigger to send an email to the Sales Manager when an order is entered whose priority is HIGH.

CREATE TABLE Orders

(Order_ID int IDENTITY,

Order_Priority varchar(10))

CREATE TRIGGER TR_Orders_INSERT

ON Orders

FOR INSERT

AS

IF (SELECT COUNT(*) FROM inserted WHERE Order_Priority = 'High') = 1

BEGIN

PRINT 'Send an email'

END

How to know information about triggers present?

USER_TRIGGERS is the data dictionary view, which can be used to obtain information about any trigger.

DESC USER_TRIGGERS;

----------------------------------------------

NAME Type

----------------------------------------------

TRIGGER_NAME VARCHAR2(30)

TRIGGER_TYPE VARCHAR2(16)

TRIGGER_EVENT VARCHAR2(75)

TABLE_OWNER VARCHAR2(30)

BASE_OBJECT_TYPE VARCHAR2(16)

TABLE_NAME VARCHAR2(30)

COLUMN_NAME VARCHAR2(4000)

REFERENCING_NAMES VARCHAR2(128)

WHEN_CLAUSE VARCHAR2(4000)

STATUS VARCHAR2(8)

DESCRIPTION VARCHAR2(4000)

ACTION_TYPE VARCHAR2(11)

TRIGGER_BODY LONG

----------------------------------------------

How to drop a trigger

DROP TRIGGER Trigger_name

How to disable a trigger

DISABLE TRIGGER {schema name} trigger name ON {object, database or ALL server}

-K Himaanshu Shukla...

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