Creating Functions, Stored Procedures, and Triggers in .NET framework

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Creating Functions, Stored Procedures, and Triggers
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Let s look at an example of how to use triggers. Human Resources has a strict policy that requires any changes to an employee s pay rate to be audited. The audit must include prior pay rate, current pay rate, the date the change was made, and the name of the person who made the change. You could accomplish the audit process within an application, but you cannot guarantee that all pay rate changes take place through applications that you control. So you decide to implement a trigger on the Employee table that fires on an UPDATE operation and logs pay-rate audit information into the EmployeeAudit table:
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CREATE TRIGGER tu_employeepayaudit ON dbo.Employee FOR UPDATE AS DECLARE @now DATETIME
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SET @now = getdate() BEGIN TRY INSERT INTO dbo.EmployeeAudit (RowImage, PayRate, ChangeDate, ChangeUser) SELECT 'BEFORE', INSERTED.PayRate, @now, suser_sname() FROM DELETED INSERT INTO dbo.EmployeeAudit (RowImage, PayRate, ChangeDate, ChangeUser) SELECT 'AFTER', INSERTED.PayRate, @now, suser_sname() FROM INSERTED END TRY BEGIN CATCH --Some error handling code ROLLBACK TRANSACTION END CATCH
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Recursive and Nested Triggers
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Because triggers fire in response to a DML operation and can also perform additional DML operations, there is the possibility for a trigger to cause itself to fire or to fire additional triggers in a chain. A trigger causing itself to fire is called recursion. For example, suppose that an UPDATE TRIGGER is created on the Customers table that modifies a column in the Customers table. The modification in the trigger causes the trigger to fire again. The trigger modifies the Customers table again, causing the trigger to be fired yet again. Because this recursion can lead to an unending chain of transactions, SQL Server has a mechanism
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Lesson 3: Implementing Triggers
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to control recursive triggers. The RECURSIVE_TRIGGERS option of a database is normally set to OFF, preventing recursion by default. If you want triggers to fire recursively, you must explicitly turn on this option.
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INSTEAD OF triggers
An INSTEAD OF trigger does not fire recursively.
Recursion can also occur indirectly. For example, suppose that an UPDATE operation on the Customers table causes a trigger to fire to update the Orders table. The update to the Orders table then fires a trigger that updates the Customers table. Indirect recursion is a subset of the cases referred to as nested triggers. The most general case of nested triggers is when a trigger makes a change that causes another trigger to fire. By setting the NESTED TRIGGERS option to 0 at the server level, you can disable all forms of nested triggers.
DDL Triggers
New in SQL Server 2005 is the ability to create triggers for DDL operations, such as when a table is created, a login is added to the instance, or a new database is created. The main purposes of DDL triggers are to audit and regulate actions performed on a database. DDL triggers let you restrict DDL operations even if a user might normally have the permission to execute the DDL command. For example, you might want to prevent anyone, including members of the sysadmin fixed server role, from altering or dropping tables in a production environment. You can create a DDL trigger for the ALTER TABLE and DROP TABLE events that causes the commands to be rolled back and a message returned telling the users that approval is needed before they can alter or drop the table. The general syntax for creating a DDL trigger is as follows:
CREATE TRIGGER trigger_name ON { ALL SERVER | DATABASE } [ WITH <ddl_trigger_option> [ ,...n ] ] { FOR | AFTER } { event_type | event_group } [ ,...n ] AS { sql_statement [ ; ] [ ,...n ] | EXTERNAL NAME < method specifier > <ddl_trigger_option> ::= [ ENCRYPTION ] [ EXECUTE AS Clause ] <method_specifier> ::= assembly_name.class_name.method_name
[ ; ] }
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Creating Functions, Stored Procedures, and Triggers
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Event groups
You can find the events that are valid for DDL triggers in the SQL Server 2005 Books Online article Event Groups for Use with DDL Triggers.
An example of a DDL trigger to prevent the dropping or altering of a table is as follows:
CREATE TRIGGER tddl_tabledropalterprevent ON DATABASE FOR DROP_TABLE, ALTER_TABLE AS PRINT 'Tables cannot be dropped or altered!' ROLLBACK ;
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