Download pdf - Session - Triggers

7/31/2019 Session - Triggers

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Implementing

Triggers


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Session Objectives

Define triggers List the uses of triggers

List the guidelines for creating triggers Explain the CREATE TRIGGER T-SQL

statement List the types of triggers

Explain INSERT triggers Explain UPDATE triggers Explain DELETE triggers


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Session Objectives (contd.)

Identify SQL statements that cannot be usedin triggers

Define nested and cascading triggers

Explain Instead Of triggers

List the factors that affect the performance of

triggers


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Triggers

Allen

Brian

Cathy

Derek

Greg

Mark

Carl

Kathy

Derek


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Triggers

Allen

Brian

Cathy

Derek

Greg

Mark

Carl

Kathy

Trigger Fired

Derek

Inserted


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Uses of Triggers

Compare versions of data

Read data from other tables in other

databases

Cascade changes or deletes throughoutrelated tables in a database

Reverse invalid changes Enforce more complex restrictions than

CHECK constraints

Execute local and remote stored procedures


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Creating Triggers

Syntax

CREATE TRIGGER Trigger_name

ON table

FOR {[DELETE] [,] [INSERT][,][UPDATE]}

[WITH ENCRYPTION]

AS Sql_statement


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Inserted & Deleted Tables

Logical tables that are accessible to triggers

Contain images of data prior to and after the

updation

Schema identical to the table being updated


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Considerations

A trigger can be associated with three actionsperformed on a table: INSERT, UPDATE, and

DELETE. A trigger applies to a single table.

The WITH ENCRYPTION option can hide a

trigger definition. An encrypted trigger can,however, not be decrypted.

A trigger can reference a view or a temporarytable, but cannot be associated with it.


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Considerations (contd.)

A trigger can include any number of SQLstatements.

By default, the permission to create thetrigger is available only to the owner of thedatabase. This permission is not transferable.

A trigger can be created only in the currentdatabase. However, it can refer to the objectsof another database.


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Types of Triggers


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Insert Trigger

CREATE TRIGGER CheckFare ON flight_details FOR INSERT AS

IF (SELECT fare FROM INSERTED AS i JOIN flight AS fON i.aircraft_code = f.aircraft_code JOIN airlines_master AS am

ON f.aircode = am.aircode WHERE i.class_code = 'FC'

AND am.airline_name = 'Indian Airlines') > 8000

BEGIN

PRINT 'CheckFareTrigger: Fare for FC class of Indian Airlines

flights cannot exceed 8000'

PRINT 'Change the fare to a value less than 8000'

ROLLBACK TRANSACTION

END

Example


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Insert Trigger (contd.)


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Update Triggers


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Column level Update trigger

CREATE TRIGGER NoUpdateMealcode

ON Meal

FOR UPDATE AS

IF UPDATE (Meal_code)

BEGIN

PRINT 'You cannot modify the meal codes'


END

Example


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Table level Update Trigger

CREATE TRIGGER NoUpdateSeats

ON Reservation

FOR UPDATE AS

IF (SELECT no_of_seats FROM inserted) > 5

BEGIN

PRINT 'You cannot book more than 5 seats'


END

Example


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Table level Update Trigger (contd.)


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Delete Trigger

CREATE TRIGGER NoDeleteBA01

ON FlightFOR DELETE AS

IF (SELECT aircraft_code FROM deleted) = BA01

BEGIN

PRINT You cannot delete the details of aircraft

code BA01


END

Example


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Delete Trigger (contd.)


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Statements not allowed in Triggers


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Nested Triggers

A trigger initiates another trigger

Nesting is permitted upto 32 levels

The @@nestlevel system global variableindicates the current nesting level

sp_configure enables or disables nesting of

triggers


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INSTEAD OF Triggers

Contain code that replaces the original datamanipulation statement

Useful for data modifications through non-updateable views

Can only be based on one data modification

action


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INSTEAD OF Trigger on view (contd.)

CREATE TRIGGER del_service

ON service_view

INSTEAD OF DELETE

AS

DELETE service WHERE service_code IN

(SELECT scode1 FROM DELETED)

DELETE airline_service WHERE service_code IN

(SELECT scode2 FROM DELETED)


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Performance of Triggers

Overhead associated with triggers is usuallylow

A significant amount of time is spent inreferencing tables other than logical tables

Deleted and inserted logical tables are always

present in memory