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How to Write a DML Trigger

Louis Davidsondrsql.org

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Agenda

• Introduction• Trigger Coding Review• Designing a Trigger Solution• Summary

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Attention: There Is Homework(lots of it)

• I can’t teach you everything about DML triggers in 1 hour• There is a plethora of code the

comes with the download• It will get you started, but is

only just the tip of the iceberg

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INTRODUCTIONA basic introduction to trigger concepts

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What are DML Triggers?

• Coded modules that are very similar to stored procedures–Cannot be called directly–“Triggered” by an INSERT, UPDATE, or DELETE–With “special” tables to access event data

• Triggers existed in Microsoft SQL Server 1.0 (far before check constraints!)

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DML Trigger Execution• Execute once per DML statement– Access the current state using INSERTED virtual object, removed rows via DELETED (Updates via

both)– Work very well on limited cardinality, OLTP-esque types of modifications

• Should not seen and not heard unless they find something wrong– Don’t return results from triggers

• 2005-Later has “disallow results from triggers” server configuration• Ability to do so will be removed in an upcoming SQL Server version

– Caveat: returning results can be effective for debugging• Execute as part of the operation statement/transaction – ROLLBACK in the trigger will stop the operation (and anything else that is part of the current

transaction)– RAISERROR/THROW in trigger will make the transaction non-commitable

• Can use EXECUTE AS to elevate the permissions of the trigger code similar to stored procedures– Only in extreme circumstances!

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DML Triggers – Two types

• INSTEAD OF – When an INSERT, UPDATE or DELETE occurs, instead of the typical code executed, the trigger executes instead. You have to code the effective INSERT, UPDATE or DELETE .– They are the first thing executed–Can be applied to view objects

• AFTER – When an INSERT, UPDATE or DELETE occurs, the typical operation occurs, and then the coded object executes after everything else.

• The use cases for each are different, which we will cover in a bit more detail later when we discuss designing a trigger solution

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Multiple Triggers

• INSTEAD OF - Each table can have only 1 for each of the operations (Maximum of 3, for INSERT, UPDATE, DELETE)• AFTER–You can have “any” number of after triggers–You can only control the first and last trigger for an operation using

sp_settriggerorder

• Caution: More triggers is not necessarily more better

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TRIGGER CODING TEMPLATE OVERVIEW

The framework to start a trigger with…

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This session is not entitled:

How to Write DML Triggers To Implement all of your Data

Integrity Needsfor a reason…

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Triggers are…

• Harder to get right than normal DDL solutions• Slower to operate than normal DDL solutions• Harder to support than normal DDL solutions

• Sometimes all we have to work with and then very very useful–Because what do customers care about?

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Top Issues with Database Implementations

• #2 - Tie –Performance –Usability

• #1 Data Quality

• Anything we can do to protect the quality of the data worth the effort (and COST)• Every tool we have in SQL Server for data integrity has at least

some use

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TRIGGER CODING REVIEWWhat makes triggers different from stored procedures

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Core Trigger Validation Patterns• Negative – Look for any bad row

IF EXISTS ( SELECT * FROM INSERTED WHERE DataIsBad = 1) THROW 50000, N'bad data exists',1;

• Positive – Count that all modified rows are correct DECLARE @rowcount1 = ( SELECT count(*) FROM INSERTED WHERE DataIsBad IS NULL…) DECLARE @rowcount2 = ( SELECT count(*) FROM INSERTED WHERE DataIsBad = 0) IF @rowsAffected <> @rowcount1 + @rowcount2 THROW 50000, N'try again!',1;

– Typical use case will include an INNER JOIN to INSERTED

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Core Trigger Modifications• Basically just executing a DML statement

• Cascading operationsDELETE TableName --likely other than the triggered oneFROM Schema.TablenameWHERE EXISTS (SELECT *

FROM DELETED WHERE DELETED.Key = TableName.Key)

• Instead of Trigger ModificationsINSERT INTO TableName (Key, Column1, RowLastModifiedTime)SELECT Key, UPPER(Column1), SYSDATETIME()FROM INSERTED

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Core Trigger Modifications

• AuditingINSERT INTO TableName_AUDIT (Operation, Key,Column1, RowLastModifiedTime, AuditTime)SELECT 'UPDATE',Key, Column1, RowLastModifiedTime, SYSDATETIME()FROM DELETED

• Error handling is managed by the TRY…CATCH block around all of the modification (and validation code)

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Trigger Nesting/Recursion• When you execute a DML statement in a trigger, by default (and the most typical setting)– The trigger will nest (INSERT trigger on table A updates a row in table A and inserts a row into

table B would cause an update trigger on table A and an INSERT trigger on table B to fire if they existed)

– The trigger will not recurse (INSERT trigger on table A inserts a row into table A will not cause the trigger to refire)

• Two settings affect these conditions (with the default values)– exec sp_configure 'nested triggers',1;– alter database <DatabaseName>

set recursive_triggers off;

• There is a demo of changing this behavior in the downloads. These settings are dangerous because they can change behavior without changing code!

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Determining Columns Modified

• Use the UPDATE function– IF UPDATE(<columnName>) --Means the column was referenced in the

statement• Example:

UPDATE table1SET column1 = column1 --,column2 = column2–UPDATE(column1) -> TRUE (even though no change)–UPDATE (column2) -> FALSE

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Trigger Coding Basic Demo Setup

• Understanding multi-row operations• Error Handling

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TRIGGER CODINGBASICS (DEMO)

Demonstrating the essential trigger coding techniques…

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DESIGNING A TRIGGER

Making sure you understand what needs to be handled by the trigger before you start coding.

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Designing a Trigger

• When using constraints, there will always be a single object needed to do the entire job–Check Constraint–Foreign Key

• When building a trigger, you have to cover:–All tables that are involved with the process–All operations that might be involved• INSERT• UPDATE• DELETE

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Choosing the type of trigger to use – AFTER

• Typically used for validation and non-destructive cascading operations• Allow you to check the effects of the DML statement–You can see the state of database after the operation

• Examples–Audit Trails that work on any edition of SQL Server– Inter-row/Inter-table data validations, such as foreign keys/range

overlapping, where constraints will not work–Summary data (where heavily tested and determined to be necessary)

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Choosing the type of trigger to use – INSTEAD OF

• Typically used to change the operation in some manner, either lightly or dramatically• Also for cascade operations to avoid RI errors, like a cascade delete• Examples–Overriding format of data (formatting input, overriding user input, such as

a date and time)– Ignoring/logging for review “bad” data (high speed data entry, instrument

data)–Making multi-table views updatable using simple T-SQL–Turning a physical delete into a logical delete – …set deletedFlag = 1

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Scenario Introduction

Let’s look at 3 basic scenarios1. Maintaining a row inserted and updated time on a row2. Preventing a negative balance3. Managing an audit trail

Note: in all of these cases, the requirement we will use will be that the logic cannot be overridden.

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Maintaining a row inserted and updated time on a row

Table Involved

Table1 (Table1Key, RowCreatedTime, RowLastModifyTime)

Row Inserted

Row Updated

Row Deleted

Type of triggers: INSTEAD OF

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Preventing a Negative BalanceTables Involved

Parent Table(ParentId (not changeable), ChildValueSum (not stored))

Child Table (ChildId, ParentId FK, Value)

Row Inserted Row Inserted

Row Updated Row Updated

Row Deleted Row Deleted

Type of triggers: AFTER

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Managing an audit trailTable Involved

Table1 (Table1Key, RowCreatedTime, RowLastModifyTime)

Row Inserted

Row Updated

Row Deleted

Type of triggers: AFTER

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PRE-DEMO SUMMARY, IN CASE TIME IS NIGHTriggers are equal parts friend and foe

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TRIGGER DESIGN AND CODING SCENARIOS

Code review …

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ADVANCED TOPICSSettings and metadata to fully understand trigger operation

Note: This section may not be achievable in a 90 minute session but will be available to download with examples

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Advanced Topics To Cover (Demos)

• Getting trigger metadata - queries• Multiple triggers of the same type on the same table and ordering• Trigger Nesting/Recursion• System Settings - can change trigger execution without changing code– sp_serveroption— nested triggers (default ON)– Determines if a DML statement from

one trigger causes other DML triggers to be executed– database option—RECURSIVE_TRIGGERS (default OFF)– Determines if an update on the

table where the trigger fired causes the same triggers to fire again– sp_serveroption–disallow results from triggers (default OFF): Turn this setting on will

ensure that any trigger that tries to return data to the client will get an error – sp_serveroption-server trigger recursion (default ON) – Determines if DDL in a server

DDL trigger causes it to fire again

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ADVANCED TOPICS (DEMO)

Coded examples showing some advanced trigger concerns

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Conclusion

• Triggers are no one’s favorite tool• They are sneaky and tend to complicate support, testing,

maintenance, etc• But that sneakiness makes them powerful

• Use sparingly, whenever necessary

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Questions? Contact info..

• Louis Davidson - louis@drsql.org• Website – http://drsql.org Get slides here• Twitter – http://twitter.com/drsql

• SQL Blog http://sqlblog.com/blogs/louis_davidson

• Simple Talk Blog – What Counts for a DBAhttp://www.simple-talk.com/community/blogs/drsql/default.aspx