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Enforcing Business Logic

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What is Business Logic?A non-technical term for the rules associated with

the data in a database that encode businesspolicies.

E.g. Automatically adding late fees for overdue items

It is in addition to input/output data validationprovided by constraints (such as check, not null,

unique, primary key, foreign key) and data types(such as int, datetime) to ensure data is consistent

Triggers, along with stored procedures can beused to implement business logic

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Business Logic is specific to the

company The Northwind Traders sample database contains

the sales data for a company called Northwind Traders,which imports and exports speciality foods fromaround the world.

The business rules for Northwind may be verydifferent from the business rules for a local trading

company or a global trading company which has adifferent business model to Northwind Traders

The developer would be told which business rulesneed to be implemented

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Triggers and ConstraintsConstraints are associated with a specific ------ within a table foreign key constraints reference the ------- --- of another table.

are suitable for simple data validation.Triggers can be programmed to take data from a number of tables,

manipulate that data, insert, update or delete data in manytables

are pieces of code that look at lot like stored procedures but areattached to tables and views

act only when the appropriate action (such as INSERT, UPDATE,or DELETE) for which the trigger has been set up has beenexecuted. You can't fire a trigger directlyonly the triggeringaction can do that.

Primary Key

Column

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Trigger or Constraint?1. Creating Audit trails to track users and the tables they

have modified

2. Automatically updating Quantity in Stock

3. Ensuring a phone number is entered using the correctformat

4. Ensuring a customer cannot be deleted if they haveorders recorded in the orders table

5. Providing an error message if the credit limit isexceeded by a customer order and blocking thetransaction

6. Uniquely identify a row of data

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Trigger

Trigger

Check Constraint

FK Constraint

Trigger

PK Constraint

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Triggers Triggers, along with stored procedures can be used to

implement business logic

Triggers were used to implement referential integrity

but that is now done using foreign key constraints Triggers were used to update CHILD tables when an

update or delete was applied to a PARENT table butthat is now done using CASCADES

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CASCADE a FK property

Use EM

Select a FK

Check the

appropriateCASCADEUPDATE orCASCADEDELETE inproperties

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What will the CASCADE do? If we select 'CascadeUpdate Related Fields' inthis property box, what

will it do?

Draw the relationship for

these tables

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It will update the ProductID in the

Order Detail Table if it is modified

in the Products table

OrderDetails

Products

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In 2008 The same

principle appliesbut the

properties havebeen presentedin a differentformat

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When would you use it in

Northwind?When would a

CASCADE UPDATE beuseful?

When might aCASCADE DELETE bedangerous?

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An 'INSTEAD OF' trigger

may be safer - we will look

at types of trigger next

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Types of trigger AFTER ( or FOR ) The

trigger actions happenAFTER the INSERT,

UPDATE or DELETE You can have multiple

'after' triggers on a table

They execute in the order

they were created

INSTEAD OF Trigger An different action happens

INSTEAD of the INSERT,

UPDATE or DELETE can be used to perform

validation before the data isaltered in the table

used to implement updates

on Views which wouldnormally not be updateable

only 1 INSTEAD OF triggeris allowed per process

is executed before any

AFTER triggers 11

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Trigger SyntaxAFTER ( or FOR )

CREATE TRIGGER trigger_name

ON table_nameFOR DELETE, INSERT, UPDATE

AS[action that you want the trigger to

make e.g.]

INSERT into another_tableVALUES (current_user,current_timestamp)

GO

INSTEAD OF Trigger

CREATE TRIGGER trigger_nameON table_name INSTEAD OFDELETE AS

[action that you want the trigger todo instead of deleting e.g.changing the status of a columnnamed 'expired' to Y]

GO

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'After or For' TriggerCREATE TRIGGERreminder1 ON Employee

AFTER INSERT, UPDATEASPRINT 'Notify HR'GO

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INSERTED

7, '25- 12- 03', 'Smith, 1000

EmployeeConstraints

TRIGGERINSERT

UPDATE

DELETE

7, '25- 12- 03', 'Smith, 1000

Notify

HR

Insert into employee values (7, '25- 12- 03', 'Smith, 1000)

notify

HR

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'After or For' TriggerCREATE TRIGGER SalIncrease ON EmployeeAFTER UPDATEASif (SELECT SALARY FROM INSERTED)>1.1* (SELECT SALARY FROM DELETED)

BEGINROLLBACK TRANSACTION

RAISERROR ('TOO MUCH', 16, 10)

END

GO

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DELETED

7, '25- 12- 03', 'Smith, 1000

EmployeeConstraints

TRIGGERINSERT

UPDATE

DELETE

7, '25- 12- 03', 'Smith, 1000

TOO

MUCH

UPDATE employee SET SAL= 1300 WHERE EMPID=7

TOO

MUCH

INSERTED

7, '25- 12- 03', 'Smith, 1300 7, '25- 12- 03', 'Smith, 13007, '25- 12- 03', 'Smith, 1000

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Instead of' TriggerCREATE TRIGGER UrFiredON EmployeeINSTEAD OF DELETEASUpdate Employee SET EmpEnded = Y WHERE EmpID=(Select EmpID from Deleted)GO

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DELETED

7, '25- 12- 03', 'Smith, 1000

EmployeeConstraints

TRIGGER

INSERT

UPDATEDELETE

7, '25- 12- 03', 'Smith, 1000

DELETE FROM employee WHERE EMPID = 7

7, '25- 12- 03', 'Smith, 1000, Y

The Employee table has been updated

instead of deleted so what will the updateevent have triggered?

SalIncrease trigger (previous slide) will fire and

check the salary column, but as the update do

infringe the conditions no action will be seen

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Examples In the following slides examine the trigger codes then

say what action will occur for each event

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Make a note of these triggersA

B

C

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When the following transactions run what actions

occur and why? 1

2

3

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1.Welcome to the team

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2. For update

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3. Sorry you are leaving

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4. Data type property infringed The EmployeeId field is set as integer data type with

the properties set to IDENTITY

This automatically inserts an EmployeeID and meansthat you cannot insert your own value

MS SQL Server prevents the insert so the trigger isnever executed

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Triggers can be used for:

Single table complex data validation. What does this do?CREATE trigger GotEnough

ON [Order details]

For INSERT, UPDATE

ASif (SELECT Quantity FROM INSERTED)>

(SELECT UnitsInStock

FROM Products p Join INSERTED i ON p.product_ID =i.Product_ID )

BEGIN

ROLLBACK TRANSACTION

RAISERROR ('Not Got Enough in Stock', 16, 10)

END

Find theQuantity in Stock

in PRODUCTS

How many we have ordered in

the new or updated order

detail record which is held

temporarily in INSERTED

by taking the

ProductID held in

INSERTED and

matching it to the

ProductID in

PRODUCTS

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How does the join work?if (SELECT Quantity FROM INSERTED)>

(SELECT UnitsInStock

FROM Products p Join INSERTED i ON p.product_ID =i.Product_ID )

Find the

Quantity in Stock

currently in

Products

This is the quantity

we want to insert

into Order Details

by taking the

ProductID from

the inserted

table andfinding the

matching

ProductID in

the Product

Table

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What does this do?create trigger trOrderInsert

on [Order details]

for INSERT

AS

UPDATE ProductsSET p.UnitsInStock = (p.UnitsInStock - i.Quantity)

FROM Products p join Inserted i

on p.ProductID = i.ProductID

Use the quantity just

inserted into the

order_details table

To change the

UnitsInStock in the

Products table

Which can only be found by

matching the productID from the

INSERTED table with that in the

PRODUCTS table

Subtract that from the

current units in stock

for the ProductID inProducts

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Triggers can be used for:

producing an audit trail in an audit table First create your table

CREATE TABLE tblAudit

(a_user char (10) ,a_date datetime ,

a_ID int IDENTITY (1, 1) NOT NULL ,

CONSTRAINT [PK_tblAudit] PRIMARY KEY(a_id))

Identity is essential forthe trigger to work

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What does this trigger do?

CREATE TRIGGER trg_audit

ON deptAFTER INSERT, UPDATE, DELETE

AS

INSERT into tblAudit VALUES (current_user,

current_timestamp)

This is a funtion that

inserts usernames

for any one with

modification

permissions who

sends an insert,

update or delete to

the DEPT table

Displays the date and

time this is a function

equivalent to

GETDATE()

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What does this trigger do?

The DBA creates the following trigger.

CREATE TRIGGER [trg_delete_check]ON [dbo].[Order Details]

AFTER DELETEASIF (SELECT COUNT(*) FROM DELETED)>1BEGIN

RAISERROR (some sort of informational warningor instruction',16,1)ROLLBACK TRANSACTIONEND

This counts the

number of records in

the DELETED table

If the total is greater

than one the delete

will be prevented and

an error message

displayed

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SummaryTriggers Can be programmed to take data from a number of tables,

manipulate that data, insert, update or delete data in manytables

Are pieces of code that look at lot like stored procedures but areattached to tables and views

Act only when the appropriate action (such as INSERT, UPDATE,or DELETE) for which the trigger has been set up has beenexecuted. You can't fire a trigger directlyonly the triggeringaction can do that.

Triggers take values from the temporary tables INSERTED andDELETED for any values needed in join statements, conditionsetc used in the trigger code

INSTEAD OF triggers fire before any AFTER/FOR triggers You can have multiple AFTER triggers on a table