Sql xp 10

Implementing Triggers and Transactions

Objectives
In this lesson, you will learn to:
Create the INSERT, DELETE, and UPDATE triggers
Modify triggers
Drop triggers
Enforce data integrity through triggers
Use the AFTER and the INSTEAD OFF triggers

©NIIT SQL/Lesson 10/Slide 1 of 49


10.D.1 Using the INSERT Trigger
When a new row is inserted in the Requisition table, the
value of the siNoOfVacancy attribute should be less than the
difference between the iBudgetedStrength and
iCurrentStrength attributes of the Position table. Ensure that
this user-defined data integrity requirement is implemented.



Task List
Identify the object that can maintain user-defined data
integrity
Draft statements to create an INSERT trigger
Create a trigger in the database
Check the existence of the trigger in the database
Insert a row in the Requisition table and verify that the trigger
is working



Identify the object that can maintain user-defined data
integrity
A trigger is a block of code that constitutes a set of T-SQL
statements that are activated in response to certain actions
Characteristics of a Trigger
It is fired automatically by SQL Server when any data
modification statement is issued
It cannot be explicitly invoked or executed, as in the case
of the stored procedures



Identify the object that can maintain data integrity
(Contd.)
It prevents incorrect, unauthorized or inconsistent changes
in data
It cannot return data to the user
Result:
A trigger can be used to maintain data integrity



Draft the statements to create an INSERT trigger
Creating triggers
Triggers can be created in the Query Analyzer window by
using the CREATE TRIGGER statement
Syntax
CREATE TRIGGER trigger_name
ON table_name
[WITH ENCRYPTION]
FOR [INSERT | DELETE | UPDATE]
AS sql_statements



(Contd.)
Magic tables
Whenever a trigger fires in response to the INSERT,
DELETE, or UPDATE statement, two special tables are
created. These are the inserted and the deleted tables.
They are also referred to as magic tables
The inserted table contains a copy of all records that are
inserted in the trigger table
The deleted table contains all records that have been
deleted from the trigger table
Whenever any updation takes place, the trigger uses both
the inserted and the deleted tables


(Contd.)
The INSERT Trigger
An INSERT trigger is fired whenever an attempt is made to
insert a row in the trigger table
When an INSERT statement is issued, a new row is added
to both the trigger and the inserted tables



Draft the statements to create an insert trigger (Contd.)
Action:
The table on which the trigger has to be created is
Requisition
The trigger has to be of insert type
The name of the trigger can be trgInsertRequisition
Write the batch statements
CREATE TRIGGER trgInsertRequisition
ON Requisition FOR insert AS
DECLARE @VacancyReported int
DECLARE @ActualVacancy int
SELECT @ActualVacancy = iBudgetedStrength -
iCurrentStrength



Draft the statements to create an insert trigger (Contd.)
FROM Position Join Inserted on
Position.cPositionCode =
Inserted.cPositionCode
SELECT @VacancyReported =
inserted.siNoOfVacancy
FROM inserted
IF(@VacancyReported @Actualvacancy)
BEGIN
PRINT 'The actual vacancies are less than the
vacancies reported. Hence, cannot insert.'
ROLLBACK TRANSACTION
END
RETURN


Create a trigger in the database
Action:
Type the drafted code in the Query Analyzer window
Press F5 to execute the code



Action:
sp_help trgInsertRequisition



Insert a row in the Requisition table and verify that the
trigger is working
Action:
INSERT Requisition
VALUES('000003','0001',getdate(), getdate() +
7, '0001', 'North', 20)



Just a Minute...
1. What are magic tables?
2. Which statement is used to create triggers?



10.P.1 Using the INSERT Trigger
When a new row is added to the Employee table, the
iCurrentStrength attribute of the Position table should be
increased by one.



10.D.2 Using the DELETE Trigger
Create a trigger to disable deleting rows from the
ContractRecruiter table



Task List
Draft statements to create a delete trigger
Create the trigger in the database
Delete a row from the ContractRecruiter table to verify the
trigger



Draft statements to create a DELETE trigger
DELETE trigger
A DELETE trigger is fired whenever an attempt is made
to delete rows from the trigger table
There are three ways of implementing referential integrity
using a DELETE trigger. These are:
® The Cascade method
® The Restrict method
® The Nullify method



Draft statements to create a DELETE trigger (Contd.)
Result:
The table on which the trigger is to be created is
ContractRecruiter
The trigger is a DELETE trigger
The name of the trigger is trgDeleteContractRecruiter
The batch statements are:
CREATE TRIGGER trgDeleteContractRecruiter
ON ContractRecruiter FOR delete
AS
PRINT 'Deletion of Contract Recruiters is
not allowed'
ROLLBACK TRANSACTION RETURN


Action:
Type the drafted code in the Query Analyzer window

Action:
sp_help trgDeleteContractRecruiter



Delete a row from the ContractRecruiter table to verify
the trigger
Action:
Execute the following statement:
DELETE ContractRecruiter
WHERE cContractRecruiterCode = '000001'
When this command is executed, the trigger would be
fired and it would prevent the deletion of rows from the
ContractRecruiter table



Just a Minute...
When is a DELETE trigger fired?



10.D 3. Using the UPDATE Trigger
Create a trigger so that the average siPercentageCharge
attribute of the ContractRecruiter table should not be more
than 11 when the value of siPercentageCharge is increased
for any ContractRecruiter



Task List
Draft statements to create an update trigger
Update siPercentageCharge of the ContractRecruiter table
and verify that the average does not exceed the required
value



Draft statements to create an UPDATE trigger
The UPDATE trigger
This trigger is fired whenever there is a modification to
the trigger table
Result:
The table on which the trigger is to be created is
ContractRecruiter
The trigger is an UPDATE trigger
The name of the trigger is trgUpdateContractRecruiter



Draft statements to create an UPDATE trigger (Contd.)
The trigger is:
CREATE TRIGGER trgUpdateContractRecruiter
ON ContractRecruiter FOR UPDATE
AS
DECLARE @AvgPercentageCharge int
SELECT @AvgPercentageCharge =
avg(siPercentageCharge)
FROM ContractRecruiter
IF(@AvgPercentageCharge 11)
BEGIN
PRINT 'The average cannot be more than 11'
END
RETURN



In the Query Analyzer window, type the draft code

Action:
sp_help trgUpdateContractRecruiter



Update siPercentageCharge of the ContractRecruiter
table and verify that the average does not exceed the
required value
Action:
Execute the following UPDATE statement:
UPDATE ContractRecruiter
SET siPercentageCharge
=siPercentageCharge+10
WHERE cContractRecruiterCode='0002’
If the average exceeds the limit, then the trigger would
generate an error message



10.P.2 Using the UPDATE Trigger
When an employee resigns, the resignation date is updated
in the Employee table. After the resignation date is updated,
the iCurrentStrength attribute of the Position table should be
decreased by 1.



10.D.4 Modifying the Trigger
Modify the trigger trgInsertRequisition that was created
earlier to check whether the siNoOfVacancy attribute is less
than the difference between iBudgetedStrength and
iCurrentStrength from the Position table. If so, the trigger
should display a message:
“Sorry, the available vacancy is less than the reported
vacancy. The transaction cannot be processed.”



Task List
Draft the command to modify the trigger
Check that the trigger has been modified in the database
Insert a row in the Requisition table and verify that the trigger
is working



Draft the command to modify the trigger
The ALTER TRIGGER Command
The contents of a trigger can be modified by:
® Dropping the trigger and recreating it
® Using the ALTER TRIGGER statement
It is advisable to drop a trigger and recreate it, if the
objects being referenced by it are renamed



Draft the command to modify the trigger (Contd.)
Syntax:
ALTER TRIGGER trigger_name
ON table_name
[WITH ENCRYPTION]
FOR [INSERT | DELETE | UPDATE]
AS sql_statements
Action:
The table on which the trigger had been created is
Requisition
Determine the type of trigger
The name of the trigger to be modified is
trgInsertRequisition



Write the batch statements:
ALTER TRIGGER trgInsertRequisition ON
Requisition
FOR insert
AS
DECLARE @VacancyReported int
DECLARE @ActualVacancy int
SELECT @ActualVacancy = iBudgetedStrength –
iCurrentStrength
FROM Position
SELECT @VacancyReported =
inserted.siNoOfVacancy
FROM inserted



IF(@VacancyReported @ActualVacancy)
BEGIN
RAISERROR ('Sorry, the available
vacancy is less than the reported vacancy.
The transaction cannot be processed.', 10,
1)
END
RETURN
Action:
In the Query Analyzer window, type the drafted code
Press F5 to execute the batch statement


Check that the trigger has been modified in the
database
Action:
sp_helptext trgInsertRequisition
Insert a row in Requisition table and verify that the
trigger is working
Action:
INSERT Requisition
VALUES('000002','0001',getdate(),getdate()
+7,’0001','North',20)



Just a Minute...
Which statement is used to recreate a trigger?



10.D.5 Dropping the Trigger
The DELETE trigger named trgDeleteContractRecruiter
needs to be removed, as it is no longer required.



Task List
Draft the command to delete the trigger
Execute the command
Verify that the trigger has been removed



Draft the command to delete the trigger
The DROP TRIGGER Command is used to delete a trigger
from the database
Syntax:
DROP TRIGGER trigger_name[,..n]
Action:
The command to delete the trigger would be:
DROP TRIGGER trgDeleteContractRecruiter



Execute the command
Action:
In the Query Analyzer window, type the drafted code
Press F5 to execute the batch statement

Verify that the trigger has been removed
Action:
sp_help trgDeleteContractRecruiter
The above command will give an error message as the
trigger has been removed



Enforcing Data Integrity Through Triggers
Triggers and Data Integrity
A trigger can be used to enforce business rules and data
integrity in the following ways:
® Ifchanges are made to the master table, then the same
changes are cascaded to all the dependent tables
® Ifsome changes violate referential integrity, then all
such changes are rejected, thereby canceling any
attempt to modify data in the database
® It allows very complex restrictions to be enforced
® Itcan perform a particular action, depending on the
outcome of the modifications that have been made to
the tables


Enforcing Data Integrity Through Triggers (Contd.)
Multiple Triggers
SQL Server allows multiple triggers to be defined on a
given table. This implies that a single DML statement may
fire two or more triggers. The triggers are fired in the
order of creation
AFTER and INSTEAD OF Triggers
The AFTER trigger can be created on any table for the
INSERT, UPDATE, or DELETE operation just like the
normal triggers
The AFTER trigger gets fired after the execution of the
DML operation for which it has been defined


By default, if more than one AFTER trigger is created on a
table for a DML operation such as INSERT, UPDATE, or
DELETE, then the sequence of execution is the order in
which they were created
In case you have multiple AFTER triggers for any single
DML operation, you can change the sequence of execution
of these triggers by using the sp_settriggerorder system
stored procedure
Syntax
sp_settriggerorder triggername,order-value,DML-
operation



INSTEAD OF triggers can be primarily used to perform an
action such as a DML operation, on another table or view
This type of trigger can be created on both table as well as
on a view
Unlike AFTER triggers you cannot create more than one
INSTEAD OF trigger for a DML operation on the same table
or view



Just a Minute...
How are triggers used to maintain integrity and consistency
of data?



Summary
In this lesson you learned that:
A trigger is a block of code that constitutes a set of T-SQL
statements that get activated in response to certain actions
A trigger fires in response to the INSERT, UPDATE, and
DELETE statements
A trigger can be created in the Query Analyzer by using the
CREATE TRIGGER statement
A magic table is a conceptual table that is structurally similar
to the table on which a trigger is defined



Summary (Contd.)
There are two types of magic tables:
Inserted, which stores a copy of the rows that have been
inserted into the trigger table
Deleted, which stores those records that have been
deleted from the trigger table
A trigger can be viewed using the sp_help and sp_helptext
system stored procedures
A trigger can be altered using the ALTER TRIGGER
statement
A trigger can be deleted using the DROP TRIGGER
statement


Summary (Contd.)
A trigger can be used to enforce business rules and data
integrity
The AFTER trigger is executed after all constraints and triggers
defined on the table have successfully executed
INSTEAD OF triggers can be used to perform another action
such as a DML operation on another table or view


Sql xp 10

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Sql xp 10