1. Responding to Data Manipulation Via Triggers
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11: Responding to Data Manipulation Via Triggers
Module 11
Responding to Data Manipulation Via Triggers

2. Lesson 1: Designing DML Triggers
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Lesson 1: Designing DML Triggers
11: Responding to Data Manipulation Via Triggers
Considerations for Triggers
Question
Which types of statements fire DML triggers?
( )Option 1: CREATE, ALTER, DROP
( )Option 2: MERGE, ALTER, DELETE
( )Option 3: MERGE, CREATE, INSERT
( )Option 4: CREATE, UPDATE, DELETE
( )Option 5: DELET, INSERT, UPDATE
Answer
(√) Option -2: DELET, INSERT, UPDATE
What reasons can you think of for deploying AFTER triggers?
Common reasons for implementing AFTER triggers are:
Providing auditing of the changes that were made.
Implementing complex rules involving the relationship between tables.
Implementing default values or calculated values within rows.

3. Triggers are special stored procedures which:
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What Are DML Triggers?
11: Responding to Data Manipulation Via Triggers
Triggers are special stored procedures which:
Fire for INSERT, UPDATE, or DELETE DML operations
Fire on DDL statements such as CREATE, ALTER, or DROP
Provide complex logic and meaningful error messages
Multiple triggers can be fired
Use this slide to discuss what a trigger is and how it executes when data is modified.
Remind the students that cascading referential integrity constraints were discussed earlier.
Don’t discuss each type of trigger, because the triggers are addressed individually in the following topics. Also, note that DDL triggers and LOGON triggers are out of scope for this course, but an example of a DDL trigger is provided in the XML modules.
More detail will be provided in later slides.

4. AFTER Triggers vs. INSTEAD OF Triggers
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AFTER Triggers vs. INSTEAD OF Triggers
11: Responding to Data Manipulation Via Triggers
Two types of triggers can be implemented in managed code or Transact-SQL:
AFTER triggers
Fire after the event to which they relate
Are treated as part of the same transaction as the statement that triggered them
Can roll back the statement that triggered them (and any transaction of which that statement was part)
INSTEAD OF triggers
Make it possible to execute alternate code, unlike a BEFORE trigger in other database engines
Are often used to create updatable views with more than one base table
Use this slide to discuss what a trigger is and how it executes when data is modified.
Remind the students that cascading referential integrity constraints were discussed earlier.
Don’t discuss each type of trigger, because the triggers are addressed individually in the following topics. Also, note that DDL triggers and LOGON triggers are out of scope for this course but an example of a DDL trigger is included in the XML modules.
More detail will be provided in later slides.

5. Inserted and Deleted Virtual Tables
11: Responding to Data Manipulation Via Triggers
Inserted and deleted virtual tables
Provide access to state of the data before and after the modification began
Are often joined to the modified table data
Are available in both AFTER and INSTEAD OF triggers
Deleted table
DELETE statements – rows just deleted
UPDATE statements – original row contents
Inserted table
INSERT statements – rows just inserted
UPDATE statements – modified row contents
Inserted and deleted virtual tables are used so that, during the execution of a trigger, you can access both the modified and unmodified versions of the data. Having access to both modified and unmodified versions often requires a join between the table data and the virtual table.
These tables only exist for the duration of the triggers and in the same scope as the triggers. For example, if the trigger calls a stored procedure, that procedure cannot see these tables.
These tables are also available in INSTEAD OF triggers.

6. SET NOCOUNT ON Triggers should not return rows of data
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SET NOCOUNT ON
11: Responding to Data Manipulation Via Triggers
Triggers should not return rows of data
Client applications often check the number of rows that are affected by data modification statements
Triggers that are affected by data modification statements
SET NOCOUNT ON avoids affecting the outer statements
Returning rowsets has been deprecated
Use the configuration setting “disallow results from triggers” to prevent triggers from returning resultsets
If a client application updates three rows of a table, it often checks the returned rowcount to make sure that only three rows are affected.
Adding a trigger to the table could cause this count to change (and the application to fail) if the trigger also performed data modification.
To avoid this, most triggers should have SET NOCOUNT ON within the trigger body. SET operations within a trigger remain in effect during the execution of the trigger, but then revert to their previous settings.
Triggers should generally never return rows of data. In future versions, the ability to do so will be removed. To prevent triggers for returning result sets in SQL Server, set the configuration setting “disallow results from triggers” to 1.
Use the example in the subject matter to describe how SET NOCOUNT ON is used.

7. Considerations for Triggers
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Considerations for Triggers
11: Responding to Data Manipulation Via Triggers
Constraints:
Are preferred to triggers
Avoid data modification overhead on violation
Triggers:
Are complex to debug
Use a rowversion store in tempdb database
Excessive usage can impact tempdb performance
Can increase the duration of transactions
Managing Trigger Security
Wherever possible, you should use constraints rather than triggers. Constraint violations avoid the statements ever occurring. Trigger rollbacks have to undo the work performed after it has been done.
In earlier versions of SQL Server, the inserted and deleted virtual tables were essentially implemented like a view over the transaction log—the data had to be reconstructed to make it available for them.
From SQL Server 2005 onwards, the inserted and deleted virtual tables have been created using the rowversion store held in tempdb. This improves trigger performance but excess trigger usage can adversely affect the performance of tempdb.
Avoid implementing too much business logic within triggers because they can be quite tricky to debug.
Ensure that the students are aware of the security issues of executing DDL and DML triggers.

8. Lesson 2: Implementing DML Triggers
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Lesson 2: Implementing DML Triggers
11: Responding to Data Manipulation Via Triggers
Demonstration: Working with AFTER UPDATE Triggers
Question
Is the following statement true or false?
“When rows are deleted from a table by using a DELETE statement, any AFTER DELETE triggers are fired when the deletion is completed. DELETE ROWS is an administrative option that removes all rows from a table.”
( )False
( )True
Answer
(√)False
Analyze this create trigger code and indicate the four errors. You can assume the table and columns have been created.
Outline some code you could use to test the trigger.
CREATE TRIGGER TR_SellingPrice_InsertUpdate
IN dbo.SellingPrice
AFTER INPUT, UPDATE AS BEGIN
SET NOCOUNT OPEN;
INSERT sp
SET sp.ExtendedAmount = sp.SubTotal
+ sp.TaxAmount
+ sp.FreightAmount
FROM dbo.SellingPrice AS sp
INNER JOIN inserted AS i
ON sp.SellingPriceID = i.SellingPriceId;
END; GO
(More notes on the next slide)

9. INSERT statement is executed AFTER INSERT trigger then fires
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AFTER INSERT Triggers
11: Responding to Data Manipulation Via Triggers
INSERT statement is executed
AFTER INSERT trigger then fires
Ensures that multirow inserts are supported
An INSERT trigger executes whenever an INSERT statement inserts data into a table or a view on which the trigger is configured.
Walk the students through the CREATE TRIGGER statement example. Explain the function of every statement within the trigger. In particular, note the use of SET NOCOUNT ON. Also note that the trigger works with multiple rows of data. It is important to avoid writing triggers that only work for a single row.

10. DELETE statement is executed AFTER DELETE trigger then fires
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AFTER DELETE Triggers
11: Responding to Data Manipulation Via Triggers
DELETE statement is executed
AFTER DELETE trigger then fires
Multirow deletes
Truncate table
A DELETE trigger is a special kind of stored procedure that executes whenever a DELETE statement deletes data from a table or a view on which the trigger is configured.
You may want to use a DELETE trigger when the record being deleted has referring records and you want to update or eliminate the references. For example, if you have an Authors table and a Books table, you may want to delete the books associated when an author is deleted. While this could be done with cascading referential integrity, you might also use the DELETE trigger to move records to an audit or archive table when they are deleted, or to execute more complex logic.
Point out to students that DELETE triggers are not executed for TRUNCATE TABLE operations.

11. UPDATE statement is executed AFTER UPDATE trigger then fires
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AFTER UPDATE Triggers
11: Responding to Data Manipulation Via Triggers
UPDATE statement is executed
AFTER UPDATE trigger then fires
Trigger processes updated rows at the same time
An UPDATE trigger executes whenever an UPDATE statement changes data in a table or a view on which the trigger is configured.
Walk the students through the CREATE TRIGGER example on the slide.

12. Lesson 3: Advanced Trigger Concepts
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Lesson 3: Advanced Trigger Concepts
11: Responding to Data Manipulation Via Triggers
Demonstration: Replacing Triggers with Computed Columns
Question
What are the four missing terms from this statement indicated by <XXX>?
<XXX>
These triggers are most commonly used to enable views that are based on multiple base tables to be updatable. You can define <XXX> triggers on views that have one or more base tables, where they can extend the types of updates that a view can support.
This trigger executes instead of the original triggering action. <XXX> triggers increase the variety of types of updates that you can perform against a view. Each table or view is limited to one <xxx> trigger for each triggering action (<XXX>).
You can specify an <XXX> trigger on both tables and views. You cannot create an <XXX> trigger on views that have the <XXX> clause defined. You can perform operations on the base tables within the trigger. This avoids the trigger being called again. For example, you could perform a set of checks before inserting data, and then perform the insert on the base table.
( )Option 1: How Nested Triggers Work; INSTEAD OF; DELETE; CHECK OPTION
( )Option 2: Updatable Views; AFTER INSERT; INSERT, UPDATE, or DELETE; NO ROWCOUNT
( )Option 3: Updatable Views; INSTEAD OF; DML or DDL; CHECK OPTION
( )Option 4: Updatable Views; RATHER THAN; UPDATE; CHECK OPTION
( )Option 5: Updatable Views; INSTEAD OF; INSERT, UPDATE, or DELETE; CHECK OPTION
Answer
(√) Option -2: Updatable Views; INSTEAD OF; INSERT, UPDATE, or DELETE; CHECK OPTION

13. INSERT, UPDATE or DELETE statement requested to be executed
INSTEAD OF Triggers
11: Responding to Data Manipulation Via Triggers
INSERT, UPDATE or DELETE statement requested to be executed
Statement does not execute
INSTEAD OF trigger code executes instead
Updatable views are a common use
An INSTEAD OF trigger is executed in place of the usual triggering action.
Walk the students through the CREATE TRIGGER statement and explain the options used. In the example, we are making sure that products are flagged as discontinued when someone tries to delete them.

14. How Nested Triggers Work
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How Nested Triggers Work
11: Responding to Data Manipulation Via Triggers
Turned on at the server level
Complex to debug
Nested triggers are ON by default and set as a server level option. Any trigger can contain an UPDATE, INSERT, or DELETE statement that affects another table. Triggers are nested when a trigger performs an action that initiates another trigger.
Triggers can be nested up to 32 levels.
Stress that nested triggers can be disabled by an administrator at the server level.

15. Considerations for Recursive Triggers
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Considerations for Recursive Triggers
11: Responding to Data Manipulation Via Triggers
Recursive triggers are disabled by default
To turn them on:
ALTER DATABASE db SET RECURSIVE_TRIGGERS ON
Direct Recursion
Indirect Recursion
Considerations:
Careful design and testing to ensure that the 32-level nesting limit is not exceeded
Difficult to control the order of table updates
Can usually be replaced by nonrecursive logic
The RECURSIVE TRIGGERS option only affects direct recursion
Use this slide to explain how recursive triggers happen and the important considerations. Mention that nested triggers are a server option, which is ON by default, and recursive triggers are a database option, which is OFF by default.
A recursive trigger performs an action that causes the same trigger to fire again, either directly or indirectly—just as in the previous slide, except that the trigger points back to the same table (direct) or a trigger fired from another table as a result points back to the table (indirect).
You can use the RECURSIVE_TRIGGERS database option to enable or disable direct recursion in triggers.
You use the SQL Server nested triggers configuration to enable or disable indirect recursion.

16. UPDATE function—is a column being updated?
11: Responding to Data Manipulation Via Triggers
UPDATE function—is a column being updated?
Used in AFTER INSERT and AFTER UPDATE
COLUMNS UPDATED function returns bitmap of columns being updated
It is important to note that the value is not related to whether the value in a column is being updated, only to whether or not the column is one of the columns that is being updated. Even if a value is being changed to the same value it already has, it will be included by this function.
Mention that the COLUMNS_UPDATED function can be used to retrieve a bitmap indicating which columns are being updated. However, it is much harder to use than the UPDATE function and depends upon the positional information for the column. Hard-coding that positional information in a trigger affects the maintainability of your trigger code. It also makes the code far less readable.

17. Firing Order for Triggers
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Firing Order for Triggers
11: Responding to Data Manipulation Via Triggers
Multiple triggers may be created for a single event
You cannot specify the order in which the triggers will fire
With sp_settriggerorder, you can specify which triggers will fire first and last
In general, you should not depend on the order that triggers fire in. SQL Server determines the order.
sp_settriggerorder does provide some control over this by specifying which triggers fire first and last.
If you specify both first and last, the triggers must be different.
@order can be First, Last, or None. None is the default and specifies undefined order.
@stmttype is the statement type and can be INSERT, UPDATE, or DELETE for DML triggers.

18. Alternatives to Triggers
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Alternatives to Triggers
11: Responding to Data Manipulation Via Triggers
Many developers use triggers in situations where alternatives would be preferable:
Use constraints for checking values
Use defaults for values not supplied during inserts
Use foreign key constraints to check for referential integrity
Use computed and persisted computed columns
Use indexed views for precalculating aggregates
Many database developers use triggers to implement most of their integrity checking. In general, this is not a good practice.
Stress that triggers are not the answer to every problem. Instead, you should use the appropriate object that is designed for the task.
For example, many users create triggers to maintain columns that could be computed columns. Computed columns are a better option for this. In addition, computed columns are less likely to be disabled than triggers.