Data Manipulation Language (DML) — Unlike SP’s and Functions DML Triggers are not stand-alone objects, and you cannot directly execute them. A DML trigger is attached to a specific table or view and defined for a particular event. The events that can cause a DML Trigger to fire are Insert, Update, and Delete. Triggers can fire in two different modes:  AFTER and INSTEAD OF. In AFTER mode the trigger will be fired after the event that started the trigger is commpleted. In INSTEAD OF mode the trigger will be fired instead of the event’s code. The most common use of a INSTEAD OF trigger is to update the base tables through a view. You can only define one INSTEAD OF trigger for each INSERT, UPDATE, or DELETE event for a view or table. 

The code within a a trigger can have any statements and constructs valid for a batch except for the following: 

Database cannot be created, altered, dropped, backed up, or restored. 

Structural changes cannot be made to the table that cause the trigger to fire such as CREATE/ALTER/DROPINDEX,  ALTER/DROPTABLE, etc… 

Triggers have access to two special tables that are dynamically generated, the INSERTED and DELETED tables. The structure of the the INSERTED and DELETED tables excactly matches the column definition of the table on which the trigger was created. This means that you can reference columns by using the same name as the table for whith the trigger was defined. When you execute an INSERT operation, the INSERTED table contains each row that was inserted into the table, whereas the DELETED table does not contain any rows. When you execute a DELETE statement, the DELETED table contains each row that was deleted from the table, and the INSERT table doesn’t contain any rows. When you execute and update statement the INSERTED table contains the after image of each row you updated. 

Some syntax rules for a DML are below: 

•  Every trigger must have a name that conforms to the rules for object identifiers.
•   You use the ON clause to specify the table or view that the trigger will be created against.
•   Using the WITH clause you can specify whether the code will be encrypted when executing and specify an  execution context.
•  The FOR clause specifies whether or not the trigger is an AFTER or INSTEAD OF as well as events that cause the trigger to fire.
•  Following the AS clause you specify the code that you want to execute when the trigger is fired.

Now with that said lets look at and example:  HR as a strict policy that requires any changes to an employees pay rate to be audited. The audit must include the following:  

•  Prior Pay Rate
• Date the change was made
• Current Pay Rate
• Name of the person who made the change

Here is and example code of how to achieve this:

CREATE TRIGGER tu_employeepayaudit
ON dbo.Employee
FOR UPDATE
AS

DECLARE @now DATETIME

SET @now = GETDATE()

BEGIN TRY
 INSERT INTO dbo.EmployeeAudit
 (RowImage, PayRate, ChangeDate, ChangeUser)
 SELECT ‘BEFORE’, DELETED.PayRate, @now, suser_sname()
 FROM DELETED

 INSERT INTO dbo.EmployeeAudit
 (RowImage, PayRate, ChangeDate, ChangeUser)
 SELECT ‘AFTER’, DELETED.PayRate, @now, suser_sname()
 FROM INSERTED
END TRYBEGIN CATCH
 –You can place some error handling code here
 ROLLBACK TRANSACTION
END CATCH

S.P.’s can return scalar values or actual result sets and are the primary interface that applications should use to access any data within a database. S.P.’s you the ability to control access to a database as well as gives you an easy way to maintain your code if changes need to be made to an application in the future.

Each procedure must have a unique name within the database. You can specifiy input paramaters and output parameters for the procedure. Inputs will be used as local variables within the procedure, and Outputs will be passed back to the routine that called the procedure.

Applying security within SQL server using stored procedures provides a very powerful secuirty mechanism. Any user granted execute permissions on a stored procedure is automatically delegated permissions to the object and commands referenced inside the stored procedure based on the permissoin set of the user who created the stored procedure.  If all data access (insertions, deletions, updates, or selects) were performed through stored procedures, users could not directly access any table in the database. Only by executing stored procedures would users be able to manage the database.

A Microsoft best practice is to use the minimum amount of code that also minimizes the amount of work performed by the system.

Now lets practice. The practice uses the AdventureWorks sample database which can be downloaded from the web.

We want to create a stored procedure to update the hire date of employees.

CREATE PROCEDURE dbo.usp_UpdateEmployeeHireDate
AS
DECLARE @now DATETIME

–Above creates a store procedure and declares a DATETIME value to be used within the procedure.

SET @now = GETDATE()

–Above sets the current date to the value.

UPDATE HumanResources.Employee
SET HireDate = @now

–Above updates the Employee tables hiredate to the value.

Chelsea Beat Portsmouth in the FA Cup today.  Didnt get to watch this becuas I was at work but glad to see that we are back to our winning ways.

There are two types of UDFs, Scalar; which return a scalar value result. And table-valued functions, which return in the form of a table result

These two types can either be what  is called Deterministic or Nondeterministic. Deterministc meaning that everytime the function is used the same value will be returned. Nondeterministic is the exact opposite in that it will return a different value everyime it is called.

Now lets practice.  The practice uses the AdventureWorks sample database which can be downloaded from the web.

We want to create a scalar function to return the model name for a prodruct given a particular product ID.  Copy and paste the code into SSMS to test it out.

USE AdventureWorks

GO

CREATE FUNCTION dbo.GetModelNameForProduct (@ProductID int)
RETURNS nvarchar(50)
–Above Creates the function name and the specifies the input parameter
–as well as defining the output variable.
WITH EXECUTE AS CALLER
–Above Specifies the statements inside the module are executed in the
–context of the caller of the module.
AS
BEGIN
 DECLARE @ModelName nvarchar(50)

 SELECT @ModelName = Production.ProductModel.Name
 FROM Production.Product INNER JOIN Production.ProductModel
   ON Production.Product.ProductModelID = Production.ProductModel.ProductModelID
 WHERE Production.Product.ProductID = @ProductID
 RETURN(@ModelName)
END;
–Above declares a variable “ModelName” which is used in the query to return the ProductID
–that is passed as the input when the function is called.
GO

SELECT dbo.GetModelNameForProduct(710);
–The above select statement will test the function out.  Run using different inputs to test.

Microsoft Technical Specialist…

I have currently been studying to pass Microsoft Exam 70-431.  Microsoft SQL Server 2005 – Implementation and Maintenance is what the test covers.   However my motivation to really take a deep dive into the book has been slack. So I decided to study through the Microsoft training kit and post on my blog certain sections that interest me.

Stay tuned for some blog entries that include my study notes as well as links to helpful resources on the net.

and

Roxy

So I am going to have a new family in about a month.  Morgan and I will be married and we have a lil girl named roxy.

Every year at this time a new CD comes out by tiesto…In Search of Sunrise 7; Alot of people i know dont listen to music like this, but i am seriously addicted to it; and i dont know why!

Check is out…  http://www.tiesto.com/

My manager sends out an update of what we are doing this week in out department.  He always ends it with some type of quote and I thought that this one was kind of cool because I understand it but alot of people probably will not. 

“There are 10 types of people in this world, those who understand binary and those who do not.”