Module 13 Implementing Triggers

8/9/2019 Module 13 Implementing Triggers

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Module 13:Implementing Triggers

Vidya Vrat Agarwal. | MCT, MCSD


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Overview

Introduction to Triggers

Defining Triggers

How Trigger Works

Performance Considerations


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Triggers are a special class ofstored procedure definedto execute / fired automatically when a DMLStatement INSERT, UPDATE or DELETE is issuedagainst a table or view.

Triggers are powerful tools that can be used to enforcebusiness rules automatically when data is modified.

Triggers can extend the integrity checking logic of SQLServer constraints, defaults, and rules, althoughconstraints and defaults should be used instead

whenever they provide all the needed functionality.

1 Table can contain up to 2 Billion Triggers.


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What Is a Trigger

Associated with a Table

Invoked Automatically

Cannot Be Called Directly

Is a Transaction

Trigger can not accept parameters


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Uses of Triggers

Cascade Changes Through Related Tables ina Database

Enforce More Complex Data Integrity Than aCHECK Constraint

Define Custom Error Messages


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Considerations for Using Triggers

Triggers Are Reactive; Constraints Are Proactive

Constraints Are Checked First

Tables Can Have Multiple Triggers for Any Action

Table Owners Must Have Permission

Cannot Create Triggers on Views or Temporary Tables

Triggers Should Not Return Result Sets

Triggers Can Handle Multi-Row Actions

Can be Nested up to 32 levels


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Creating Triggers

CREATE TRIGGER Statement

Create Trigger t_tdate

ON tdate

FOR Deleteas

set nocount on

delete from tdate

begin

print ' You can not delete from table tdate'

rollback transaction

end

Create Trigger t_tdate

ON tdate

FOR Deleteas

set nocount on

delete from tdate

begin

print ' You can not delete from table tdate'rollback transaction

end

Table Name

Trigger Action


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How Triggers Work

How an INSERT Trigger Works

How a DELETE Trigger Works

How an UPDATE Trigger Works


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When an insert trigger is fired, new rows are added toboth the Trigger Table and the Inserted Table.

The Inserted Table is a logical table that holds the copyof the rows that have been inserted.

The Inserted Table contains the logged insert activity

from the Insert Statement.


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INSERT statement to a table with an INSERT Trigger DefinedINSERT [Order Details] VALUES

(10525, 2, 19.00, 5, 0.2)

INSERT [Order Details] VALUES

(10525, 2, 19.00, 5, 0.2)

Order DetailsOrder DetailsOrder Details

OrderIDOrderID

10522

10523

10524

10522

10523

10524

ProductIDProductID

10

41

7

10

41

7

UnitPriceUnitPrice

31.00

9.65

30.00

31.00

9.65

30.00

QuantityQuantity

7

9

24

7

9

24

DiscountDiscount

0.2

0.15

0.0

0.2

0.15

0.0

519.002 0.210523

Insert statement logged

insertedinsertedinserted

1052310523 22 19.0019.00 55 0.20.2

TRIGGER Actions Execute

Order DetailsOrder DetailsOrder Details

OrderIDOrderID

10522

10523

10524

10522

10523

10524

ProductIDProductID

10

41

7

10

41

7

UnitPriceUnitPrice

31.00

9.65

30.00

31.00

9.65

30.00

QuantityQuantity

7

9

24

7

9

24

DiscountDiscount

0.2

0.15

0.0

0.2

0.15

0.0

519.002 0.210523

Trigger Code:

USE Northwind

CREATE TRIGGER OrdDet_Insert

ON [Order Details]

FOR INSERT

AS

UPDATE P SETUnitsInStock = (P.UnitsInStock I.Quantity)

FROM Products AS P INNER JOIN Inserted AS I

ON P.ProductID = I.ProductID

Trigger Code:

USE Northwind

CREATE TRIGGER OrdDet_Insert

ON [Order Details]

FOR INSERT

AS







ProductsProductsProducts

ProductIDProductID UnitsInStockUnitsInStock

1

2

3

4

12

3

4

15

10

65

20

1510

65

20

2 15

INSERT Statement to a Table with an INSERT

Trigger Defined

INSERT Statement Logged

Trigger Actions Executed

111

222

333


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When a delete trigger is fired, deleted rows from theaffected table are placed in the Deleted Table.

The Deleted Table is a logical table that holds a copy ofthe rows that have been deleted.

When a row is appended to the Deleted Table, it is nolonger exists in the database table; therefore Deleted

Table and database table have no rows in common.


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DELETE Statement to a table with a DELETE Trigger DefinedDELETE Statement to a table with a DELETE Trigger Defined

DeletedDeletedDeleted

44 Dairy ProductsDairy Products CheesesCheeses 0x150x15

DELETE statement logged

CategoriesCategoriesCategories

CategoryIDCategoryID

1

2

3

1

2

3

CategoryNameCategoryName

Beverages

Condiments

Confections

Beverages

Condiments

Confections

DescriptionDescription

Soft drinks, coffees

Sweet and savory

Desserts, candies,

Soft drinks, coffees

Sweet and savory

Desserts, candies,

PicturePicture

0x15

0x15

0x15

0x15

0x15

0x15

0x15CheesesDairy Products4

DELETE Categories

WHERE

CategoryID = 4

DELETE Categories

WHERE

CategoryID = 4

USE Northwind

CREATE TRIGGER Category_Delete

ON Categories

FOR DELETE

AS

UPDATE P SET Discontinued = 1

FROM Products AS P INNER JOIN deleted AS dON P.CategoryID = d.CategoryID

USE Northwind

CREATE TRIGGER Category_Delete

ON Categories

FOR DELETE

AS



ProductsProductsProducts

ProductIDProductID DiscontinuedDiscontinued

12

3

4

1

2

3

4

00

0

0

0

0

0

0

Trigger Actions Execute

2 1



DELETE Statement to a Table with a DELETE

Statement Defined

DELETE Statement Logged


111

222

333


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When an update trigger is fired on a table it take 2 steps :

1. Delete step

2. Insert Step

When an update statement is executed on a table that hasa trigger defined on it,

1. the original rows are moved into the Deleted Table

and the,

2. updated rows ( after image) are inserted into theInserted Table.


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UPDATE Statement to a table with an UPDATE Trigger Defined

UPDATE Employees

SET EmployeeID = 17

WHERE EmployeeID = 2

UPDATE Employees

SET EmployeeID = 17

WHERE EmployeeID = 2

UPDATE Statement logged as INSERT and DELETE Statements

EmployeesEmployeesEmployees

EmployeeIDEmployeeID LastNameLastName FirstNameFirstName TitleTitle HireDateHireDate

1

2

3

4

1

2

3

4

Davolio

Barr

Leverling

Peacock

Davolio

Barr

Leverling

Peacock

Nancy

Andrew

Janet

Margaret

Nancy

Andrew

Janet

Margaret

Sales Rep.

R

Sales Rep.

Sales Rep.

Sales Rep.

R

Sales Rep.

Sales Rep.

~~~

~~~

~~~

~~~

~~~

~~~

~~~

~~~

2 Fuller Andrew Vice Pres. ~~~

insertedinsertedinserted

1717 FullerFuller AndrewAndrew Vice Pres.Vice Pres. ~~~~~~

deleteddeleteddeleted

22 FullerFuller AndrewAndrew Vice Pres.Vice Pres. ~~~~~~

TRIGGER Actions ExecuteUSE NorthwindGOCREATE TRIGGER Employee_Update

ON EmployeesFOR UPDATE

ASIF UPDATE (EmployeeID)

BEGIN TRANSACTIONRAISERROR ('Transaction cannot be processed.\***** Employee ID number cannot be modified.', 10, 1)ROLLBACK TRANSACTION

USE NorthwindGOCREATE TRIGGER Employee_Update

ON EmployeesFOR UPDATE

ASIF UPDATE (EmployeeID)BEGIN TRANSACTION

RAISERROR ('Transaction cannot be processed.\***** Employee ID number cannot be modified.', 10, 1)ROLLBACK TRANSACTION

ASIF UPDATE (EmployeeID)BEGIN TRANSACTION

RAISERROR ('Transaction cannot be processed.\***** Employee ID number cannot be modified.', 10, 1)ROLLBACK TRANSACTION

Transaction cannot be processed.***** Member number cannot be modifiedTransaction cannot be processed.***** Member number cannot be modified

EmployeesEmployeesEmployees

EmployeeIDEmployeeID LastNameLastName FirstNameFirstName TitleTitle HireDateHireDate

1

2

3

4

1

2

3

4

Davolio

Barr

Leverling

Peacock

Davolio

Barr

Leverling

Peacock

Nancy

Andrew

Janet

Margaret

Nancy

Andrew

Janet

Margaret

Sales Rep.

R

Sales Rep.

Sales Rep.

Sales Rep.

R

Sales Rep.

Sales Rep.

~~~

~~~

~~~

~~~

~~~

~~~

~~~

~~~

2 Fuller Andrew Vice Pres. ~~~

UPDATE Statement to a Table with an UPDATE

Trigger Defined

UPDATE Statement Logged as INSERT andDELETE Statements


111

222

333


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Data Updates on Specific Column

Create Triggert_prodON prodFOR Updateas

IF UPDATE ( product )begin

Print Column can not be Updated'rollback transaction

end


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How an INSTEAD OF Trigger Works

Create a View That Combines Two or More TablesCREATE VIEW

Customers AS

SELECT *

FROM CustomersMex

UNION

SELECT *

FROM CustomersGer

CREATE VIEW

Customers AS

SELECT *

FROM CustomersMex

UNION

SELECT *

FROM CustomersGer

CustomersMexCustomersMexCustomersMex

CustomerIDCustomerID CompanyNameCompanyName CountryCountry PhonePhone

ANATR

ANTON

CENTC

ANATR

ANTON

CENTC

Ana Trujill

Antonio M

Centro Co

Ana Trujill

Antonio M

Centro Co

Mexico

Mexico

Mexico

Mexico

Mexico

Mexico

(5) 555-4729

(5) 555-3932

(5) 555-3392

(5) 555-4729

(5) 555-3932

(5) 555-3392

~~~

~~~

~~~

~~~

~~~

~~~

CustomersGerCustomersGerCustomersGer


ALFKI

BLAUS

DRACD

ALFKI

BLAUS

DRACD

Alfreds Fu

Blauer Se

Drachenb

Alfreds Fu

Blauer Se

Drachenb

Germany

Germany

Germany

Germany

Germany

Germany

030-0074321

0621-08460

0241-039123

030-0074321

0621-08460

0241-039123

~~~

~~~

~~~

~~~

~~~

~~~

INSTEAD OFtrigger directs the

update to the base

table

CustomersCustomersCustomers


ALFKI

ANATR

ANTON

ALFKI

ANATR

ANTON

Alfreds Fu

Ana Trujill

Antonio M

Alfreds Fu

Ana Trujill

Antonio M

Germany

Mexico

Mexico

Germany

Mexico

Mexico

030-0074321

(5) 555-4729

(5) 555-3932

030-0074321

(5) 555-4729

(5) 555-3932

~~~

~~~

~~~

~~~

~~~

~~~Original Insert to

the CustomersView Does Not

Occur

UPDATE is Madeto the View

ALFKI Alfreds Fu Germany 030-0074321 ~~~

ALFKI Alfreds Fu Germany 030-0074321 ~~~

INSTEAD OF Trigger Can Be on a Table or View

The Action That Initiates the Trigger Does NOT Occur

Allows Updates to Views Not Previously Updateable

111

222

333


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Triggers Work Quickly Because the inserted anddeleted Tables Are in Cache

Execution Time Determined by: Number of tables that are referenced

Number of rows that are affected

Actions Contained in Triggers Implicitly Are Part of

a Transaction


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Recommended Practices

Keep Definitions as Simple as PossibleKeep Definitions as Simple as Possible

Minimize Use of ROLLBACK in TriggersMinimize Use of ROLLBACK in Triggers

Use Triggers Only When NecessaryUse Triggers Only When Necessary


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Check Your Understanding.


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Q.1 What is a Trigger.?


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Q.2. Triggers are powerful tools that can be used toenforce business rules automatically when data ismodified. ?

1. True

2. False


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Q.3. A Trigger can be Associated with a Table or View.?

1. Yes

2. No


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Q.4. Triggers Are Proactive ; Constraints Are Reactive.

1. True

2. False


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Q.5. Name the Logical Tables that the part of SQL ServerTrigger Architecture.


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Q.6. What is the role of Inserted Table when

Insert Trigger fires.


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Q.7. What is the role of Deleted Table when DeletedTrigger fires.


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Q.8. How an Update Trigger use logical tables differentlythan other DML Statements.


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Q.9. Are there other DML Statements also whichcan make a trigger to fire.

1. Yes

2. No


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Q.10. A trigger can be called directly.

1. True

2. False


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Q.11. You are a consultant tasked with troubleshooting aclient's database. The client has a trigger that is usedto enforce complex referential integrity. The triggerreferences columns in several tables and performs acascading update. The client has tried several times,but the trigger does not execute. What is the most

likely cause?

1. The trigger is not bound to a table.

2. The trigger table contains constraints that are being

violated.

3. The data is de-normalized.

4. Triggers can only reference a single table.


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Q.12. Which of the following is true about triggers?

1. Trigger is never invoked automatically .

2. Triggers cannot be nested.

3. A table cannot have multiple triggers defined for it.

4. Triggers cannot be used to pass parameters.


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Q.13. You are the administrator of a SQL Server databasenamed Purchasing. You have a DELETE triggeron theWishlist table that has been working for several months.You ask one of your junior staff members to delete therecords from the Wishlist table. He reports later that he hassuccessfully removed the records. You check the deleted

table for the records, but they are not there. What is themost likely explanation?

1. The deleted table is no longer in cache.

2. He used the truncate table statement.

3. The records were transferred to the transaction log.

4. The deleted table is corrupt.


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Q.14. You are the administrator of a SQL Server databasenamed Training. You have created an INSERT trigger thatupdates the Classes table when new records are added tothe Courses table. You want to verify that the trigger isworking properly and that the updated data is correct.What should you check?

1. Application log

2. Transaction log

3. Inserted table

4. Sysobjects table


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Q.15.A colleague asks you for advice. She is writing acomplex trigger that updates prices in the Sales tablebased on interest rate data updated in the Statistics table.Frequently when the trigger executes, it appears that nochanges are made at all. What could be the explanation?

1. She is using too many nested triggers.

2. SQL Server is timing out in the middle of the triggerexecution.

3. She is using the Rollback Transaction statement in hercode.

4. There is insufficient disk space to execute the trigger.


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Rerview


Defining Triggers

How Trigger Works



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Thank You.

Documents

Module 13 Implementing Triggers