Introduction to Triggers using SQL - AAUpeople.cs.aau.dk/~torp/Oracle/trigger.pdf · Learning Outcomes Goals Understand basic trigger mechanism Understand pros and cons of triggers

Introduction to Triggers using SQL

Kristian Torp

Department of Computer ScienceAalborg University

www.cs.aau.dk/˜[email protected]

November 24, 2011

daisy.aau.dk

Kristian Torp (Aalborg University) Introduction to Triggers using SQL November 24, 2011 1 / 63

http://www.cs.aau.dk/~torp

mailto:[email protected]

http://daisy.aau.dk

Outline

1 A Case Study: General Concepts

2 A Case Study: With Code

3 Overview and MotivationUsage

4 Trigger Types

5 Case Study: Instead of Triggers

6 Managing Triggers

7 Summary


Learning Outcomes

GoalsUnderstand basic trigger mechanism

Understand pros and cons of triggers

See procedural code in a DBMS

Why?Widely used in database applications

Widely supported in most DBMSs

NoteConcepts presented are generalCode presented is PostgreSQL specific

One is Oracle specific because not supported on PostgreSQL


Two Minute Discussion

Usage of TriggersHave you use triggers before?

Have you used triggers for deriving temporal information?

Does your organization have any policies wrt. triggers?


Outline




4 Trigger Types


6 Managing Triggers

7 Summary


Case Study: Handling Football Players

NoteKeep track of the players in all football clubs

Keep a history of where each player has been playing

At Time 10−− AaB buys the p layer Wurtzi n s e r t i n t o p layer values ’ Wurtz ’ , ’AaB ’ ) ;

Player Table Log TableName Club

Wurtz AaB

Name Club start date stop date

Wurtz AaB 10 null

NoteAll values are copied from the table player to the table log

A value is automatically provided for the column start date

A value cannot be provided for the column stop date







Wurtz AaB


Wurtz AaB 10 null










Wurtz AaB


Wurtz AaB 10 null










Wurtz AaB


Wurtz AaB 10 null





Case Study: Handling Football Players, cont.

At Time 12−− AaB buys the p layer Cacai n s e r t i n t o p layer values ’Caca ’ , ’AaB ’ ) ;


Wurtz AaBCaca AaB


Wurtz AaB 10 null

Caca AaB 12 null

NoteSimilar to the first insert

Start date is the current-time, stop date is unknown/not specified/TBD





Wurtz AaBCaca AaB


Wurtz AaB 10 null

Caca AaB 12 null







Wurtz AaBCaca AaB


Wurtz AaB 10 null

Caca AaB 12 null







Wurtz AaBCaca AaB


Wurtz AaB 10 null

Caca AaB 12 null





At Time 15−− AaB s e l l s Wurtz to FCKupdate p layer set c lub = ’FCK ’ where name = ’ Wurtz ’ ;

Player Table Log Table

Name Club

Wurtz FCKCaca AaB


Wurtz AaB 10 15Caca AaB 12 null

Wurtz FCK 15 null

NoteThe table player is naturally updated

The column stop date for the first Wurtz row is updated in the log table

A new row is entered into the log table





Name Club

Wurtz FCKCaca AaB



Wurtz FCK 15 null








Name Club

Wurtz FCKCaca AaB



Wurtz FCK 15 null






At Time 25−− AaB s e l l s Caca to OBupdate p layer set c lub = ’OB ’ where name = ’Caca ’ ;


Name Club

Wurtz FCKCaca OB


Wurtz AaB 10 15Caca AaB 12 25Wurtz FCK 15 null

Caca OB 25 null

NoteSimilar to the first update





Name Club

Wurtz FCKCaca OB



Caca OB 25 null






Name Club

Wurtz FCKCaca OB



Caca OB 25 null




At Time 27−− AaB buys Wurtz back from FCKupdate p layer set c lub = ’AaB ’ where name = ’ Wurtz ’ ;


Name Club

Wurtz AaBCaca OB


Wurtz AaB 10 15Caca AaB 12 25Wurtz FCK 15 27Caca OB 25 null

Wurtz AaB 27 null

NoteA new Wurtz row is started in the log tableStill, similar to the previous updates

Basically a delete followed by an insert





Name Club

Wurtz AaBCaca OB



Wurtz AaB 27 null







Name Club

Wurtz AaBCaca OB



Wurtz AaB 27 null







Name Club

Wurtz AaBCaca OB



Wurtz AaB 27 null





At Time 62−− Wurtz r e t i r e s as an a c t i v e f o o t b a l l p layerde le te from p layer name = ’ Wurtz ’ ;


Name Club

Caca OB



Wurtz AaB 27 62

NoteThe Wurtz row is deleted from the player table

The value column stop date is update for the last Wurtz row





Name Club

Caca OB



Wurtz AaB 27 62







Name Club

Caca OB



Wurtz AaB 27 62







Name Club

Caca OB



Wurtz AaB 27 62




Quiz: Handling Football Players

Final TablesPlayer Table Log Table

Name Club

Caca OB



Wurtz AaB 27 62

QuestionsHow to you identify the active players in the log table?

What is the interpretation of the null values in the log table?

What is the primary key of the log table?

In how many clubs can a single player be active at once?

When are rows deleted from the log table?Kristian Torp (Aalborg University) Introduction to Triggers using SQL November 24, 2011 12 / 63

Outline




4 Trigger Types


6 Managing Triggers

7 Summary


Audit Trail Example

create tab l e p layer (p l a y e r i d i n t pr imary key ,player name varchar (50) not n u l l ,d a t e o f b i r t h date ,c lub varchar (50) not n u l l ) ;

c reate tab l e p l a y e r l o g (p l a y e r i d i n t not n u l l ,player name varchar (50) not n u l l ,d a t e o f b i r t h date ,c lub varchar (50) not n u l l ,s t a r t d a t e date not n u l l ,s top date date ) ;

NoteThe primary keys are not the same in the two tables

The stop date column is nullable


Audit Trail Example

create tab l e p layer (p l a y e r i d i n t pr imary key ,player name varchar (50) not n u l l ,d a t e o f b i r t h date ,c lub varchar (50) not n u l l ) ;

c reate tab l e p l a y e r l o g (p l a y e r i d i n t not n u l l ,player name varchar (50) not n u l l ,d a t e o f b i r t h date ,c lub varchar (50) not n u l l ,s t a r t d a t e date not n u l l ,s top date date ) ;

NoteThe primary keys are not the same in the two tables

The stop date column is nullable


Insert Trigger

create or rep lace f u n c t i o n f p l a y e r i n s ( ) r e tu rns t r i g g e r as $$begin

i n s e r t i n t o p l a y e r l o g values (new . p l a y e r i d , new . player name ,new . d a t e o f b i r t h , new . club ,cu r ren t da te , n u l l ) ;

r e t u r n new ;end ;$$ language ’ p lpgsq l ’ ;

c reate t r i g g e r p l a y e r i n sa f t e r i n s e r t on p layerf o r each row execute procedure f p l a y e r i n s ( ) ;

NoteThe new syntax

The null is inserted into the stop date column

The current date is ”give me the current date”


Insert Trigger





NoteThe new syntax




Insert Trigger





NoteThe new syntax




Let Dreams Come Trough

i n s e r t i n t o p layer values(101 , ’ Wurtz ’ , ’ 1987−03−03 ’ , ’AaB ’ ) ;

i n s e r t i n t o p layer values(102 , ’ Messi ’ , ’ 1988−02−13 ’ , ’AaB ’ ) ;

update p layerset c lub = ’ Barcelona ’where player name = ’ Messi ’ ;

de le te from p layerwhere p l a y e r i d = 101;

NoteThe “clients” cannot see the derived information


Delete Trigger

create or rep lace f u n c t i o n f p l a y e r d e l ( ) r e tu rns t r i g g e r as $$begin

update p l a y e r l o gset s top date = cu r ren t da tewhere p l a y e r i d = o ld . p l a y e r i dand s top date i s n u l l ;r e t u r n o ld ;

end ;$$ language ’ p lpgsq l ’ ;

c reate t r i g g e r p l a y e r d e la f t e r de le te on p layerf o r each row execute procedure f p l a y e r d e l ( ) ;

NoteThis trigger is only fired for delete statements


Delete Trigger







Delete Trigger







Update Trigger

create or rep lace f u n c t i o n f p l aye r up d ( ) re tu rns t r i g g e r as $$begin

−− stop o ldupdate p l a y e r l o gset s top date = cu r ren t da tewhere p l a y e r i d = o ld . p l a y e r i dand s top date i s n u l l ;−− s t a r t newi n s e r t i n t o p l a y e r l o g values (

new . p l a y e r i d , new . player name ,new . d a t e o f b i r t h , new . club ,cu r ren t da te , n u l l ) ;


c reate t r i g g e r p layer upda f t e r update on p layerf o r each row execute procedure f p l aye r up d ( ) ;


Update Trigger

create or rep lace f u n c t i o n f p l aye r up d ( ) re tu rns t r i g g e r as $$begin

−− stop o ldupdate p l a y e r l o gset s top date = cu r ren t da tewhere p l a y e r i d = o ld . p l a y e r i dand s top date i s n u l l ;−− s t a r t newi n s e r t i n t o p l a y e r l o g values (

new . p l a y e r i d , new . player name ,new . d a t e o f b i r t h , new . club ,cu r ren t da te , n u l l ) ;


c reate t r i g g e r p layer upda f t e r update on p layerf o r each row execute procedure f p l aye r up d ( ) ;


Outline




4 Trigger Types


6 Managing Triggers

7 Summary


Overview

Triggers are executed implicitlyWhen insert , update, or delete statements are executed

Similar to a stored procedure, i.e., codeCan make call-outs to procedural code

Connected to a tableIn some DBMSs also on a view

Triggers are side effectsNormally considered very bad in software engineering

Triggers not part of SQL-92 first introduced in SQL-1999.Many DBMS have supported triggers for much longer therefore limitedstandard compliance


Transition Tables

A set of “internal” tables that the DBMS uses to keep track ofmodifications made by a transaction

Four transition tables for each “real” table R during the execution of atransaction Ti

Rinserted Contains the rows inserted into R during Ti

Rdeleted Contains the rows deleted from R during Ti

Rupdatedold Contains the values of updated rows before Ti

Rupdatednew Contains the values of updated rows after Ti

Rnew = Rold \ Rdeleted \ Rupdatedold ∪ Rinserted ∪ Rupdatednew


Outline




4 Trigger Types


6 Managing Triggers

7 Summary


Usage

To generate derived valuesSum of amount of order lines

To create an audit trailThink STASI

To enforce complex business rulesWhen a customer buys goods for more than 300$ then give a 10%discount

To generate statisticsHow often is a table modified

To provide event loggingEach time a new house is inserted in the for sale table notify potentialcustomers


Another Example: Compute Derived Values I

User RequirementsWants to store order and order lines

The total amount of an order is derived from order lines

The total amount is often queried

SolutionStore the derived information total amount with the order

Makes it fast to look-up orders based on total amount

Use triggers to automatically derived the total amountConvenient for the customers


Another Example: Compute Derived Values II

create tab l e oorder (oo rde r i d i n t pr imary key ,customer name varchar2 (50) not n u l l ,amount number (10 ,2 ) d e f a u l t 0 not n u l l ,

c o n s t r a i n t amount gt zero check ( amount > 0)de fe r rab le i n i t i a l l y immediate

) ;

c reate tab l e o o r d e r l i n e (oo rde r i d i n t not n u l l ,l i n e n o i n t not n u l l check ( l i n e n o > 0) ,dsc varchar2 (50) not n u l l ,q u a n t i t y i n t not n u l l check ( q u a n t i t y > 0) ,p r ice each number (6 ,2 ) not n u l l check ( pr ice each > 0 . 0 ) ,c o n s t r a i n t o l pk pr imary key ( oorder id , l i n e n o ) ,c o n s t r a i n t o l o f k f o r e i g n key ( oo rde r i d )

re ferences oorder ( oo rde r i d )) ;


Another Example: Compute Derived Values III

create or rep lace t r i g g e r set amounta f t e r i n s e r t or update or de le te on o o r d e r l i n e

f o r each rowdec lare

va l number (10 ,2 ) := 0 ;o id i n t ;

begini f i n s e r t i n g then

va l := : new . q u a n t i t y ∗ : new . pr ice each ;o id := : new . oo rde r i d ;

e l s i f updat ing thenva l := : new . q u a n t i t y ∗ : new . pr ice each −

: o ld . q u a n t i t y ∗ : o ld . pr ice each ;o id := : new . oo rde r i d ;

e l s i f d e l e t i n g thenva l := 0 − : o ld . q u a n t i t y ∗ : o ld . pr ice each ;o id := : o ld . oo rde r i d ;

end i f ;


Another Example: Compute Derived Values IV

update oorderset amount = amount + va lwhere oo rde r i d = o id ;

end ;


A Bad Example

create or rep lace t r i g g e r set amount2a f t e r i n s e r t on oorder

f o r each rowdec lare

tmp amount number ( 1 0 , 2 ) ;begin

s e l e c t sum( q u a n t i t y ∗ pr ice each )i n t o tmp amountfrom o o r d e r l i n ewhere oo rde r i d = : new . oo rde r i d ;

update oorderset amount = tmp amountwhere oo rde r i d = : new . oo rde r i d ;

end ;


Mutating Triggers

Insert trigger trigger

trigger

NoteA trigger can make an insert, this new insert may fire a trigger, etc

Cannot have cycles will cause a mutating trigger

Causes a runtime error


Mutating Triggers

Insert trigger trigger

trigger

NoteA trigger can make an insert, this new insert may fire a trigger, etc

Cannot have cycles will cause a mutating trigger

Causes a runtime error


Correct Usage

Can have multiple triggers on a tableOrder of execution cannot be specified

Can be hard to understand a database schema with many triggersA trigger cannot modify the table that it is associated with

May cause an infinite loop of trigger executions

Not for Integrity ConstraintsDo not use triggers for enforcing referential integrity!


Correct Usage

Can have multiple triggers on a tableOrder of execution cannot be specified

Can be hard to understand a database schema with many triggersA trigger cannot modify the table that it is associated with

May cause an infinite loop of trigger executions

Not for Integrity ConstraintsDo not use triggers for enforcing referential integrity!


Trigger Parts

Eventon update of <table name> ...

ConditionMust evaluate to true

ActionCode that is executed


Two Minute Discussion

Triggers vs. Application CodeWhy not move trigger logic to applications?

Discuss the pros and cons of this

Would you like to have a trigger on select statementsWhen and where could it be useful?


Outline




4 Trigger Types


6 Managing Triggers

7 Summary


Trigger Types I

{row

statement

}×

{beforeafter

}

A total of four trigger typesSome DBMSs support INSTEAD OF triggers

Very powerful, e.g., to make view updateable or hide legacy code

Before triggers can be used for preconditions

After triggers can be used for postconditions


Trigger Types II

Statement level triggers alwaysfires

Row trigger fires only if rows aremodified


Row Triggers: Setup Table

Row TriggersExecuted once for each row modified by triggering statement

If no rows modified⇒ trigger is not executed

Example (Create Table)

create tab l e x ( i i n t pr imary key ,j i n t not n u l l ) ;

Example (Populate Table)

i n s e r t i n t o x values ( 1 , 1 ) ;i n s e r t i n t o x values ( 2 , 2 ) ;i n s e r t i n t o x values ( 3 , 3 ) ;commit ;


Before Row Trigger

Example (Stored Procedure)

−− update row before t r i g g e rcreate or rep lace f u n c t i o n f x upd be fo re row ( )re tu rns t r i g g e r as $$begin

ra i se no t i ce ’ before row ’ ;r e t u r n new ;


Example (Before Row Trigger)

create t r i g g e r x upd before rowbefore update on x

f o r each row execute procedure f x upd be fo re row ( ) ;


After Row Trigger


−− update row a f t e r t r i g g e rcreate or rep lace f u n c t i o n f x u p d a f t e r r o w ( )re tu rns t r i g g e r as $$begin

ra i se no t i ce ’ a f t e r row ’ ;r e t u r n new ;


Example (After Row Trigger)

create t r i g g e r x upd a f te r rowa f t e r update on x

f o r each row execute procedure f x u p d a f t e r r o w ( ) ;


Statement TriggersStatement-Level Triggers

Executed once for each triggering statement

Executed even if no rows are modified

Example (Stored Procedure)create or rep lace f u n c t i o n f x u pd b e f o r e s tm t ( )r e tu rns t r i g g e r as $$begin

ra i se no t i ce ’ before stmt ’ ;r e t u r n new ;


Example (Before Statement Trigger)create t r i g g e r x upd befo re s tmt

before update on xf o r each statement execute procedure f x up d be fo re s t m t ( ) ;




















After Statement Trigger


create or rep lace f u n c t i o n f x u p d a f t e r s t m t ( )r e tu rns t r i g g e r as $$begin

ra i se no t i ce ’ a f t e r stmt ’ ;r e t u r n new ;


Example (After Statement Trigger)

create t r i g g e r x u p d a f t e r s t m ta f t e r update on x

f o r each statement execute procedure f x u p d a f t e r s t m t ( ) ;


After Statement Trigger


create or rep lace f u n c t i o n f x u p d a f t e r s t m t ( )r e tu rns t r i g g e r as $$begin

ra i se no t i ce ’ a f t e r stmt ’ ;r e t u r n new ;


Example (After Statement Trigger)

create t r i g g e r x u p d a f t e r s t m ta f t e r update on x

f o r each statement execute procedure f x u p d a f t e r s t m t ( ) ;


Other Types of Triggers

System Event TriggersAt database start up or shut down

When a DBMS fails

User-Event TtriggersOn logon and logoff

When DML statement executed

When DDL statement executed

NoteThese types of triggers are not part of the SQL standard!


Outline




4 Trigger Types


6 Managing Triggers

7 Summary


Introduction


Very Strong Concept

Can make all views updateable

Can make modifications easier to understand

Can wrap ugly database design, by nice design

Can make the changes work for multiple programming languages


Mini University Schema

Customer RequirementsWants to have a readable access to students status without joins

A student name must have the column name name

A student status must the column name status

Wants to be able to modify a students status using strings

Challengesname is a reserved word

status is also a name of an exiting table

Currently needs to join to get readable status of students

Currently needs to memorize the int values used for status

SolutionsCreate a view

Use instead-of-triggers on view


Create the View

Example (The View)

create or rep lace view s tu de n t s t a t us ass e l e c t s tu . s name as ”name” , s ta . dsc as ” s ta tus ”from student stu , s ta tus s tawhere s tu . s t a t i d = sta . s t a t i d

NoteThe name and status columns are in quotes

A simple view statement


Insert Function I

create or rep lace f u n c t i o n f i n s s t u d e n t s t a t u s ( )re tu rns t r i g g e r as $$

dec larev s t a t i d s ta tus . s t a t i d%type ;v s i d s tudent . s id%type ;v no numeric ;

begin−− check t h a t the s ta tus column e x i s t s i n the s ta tus tab l ebegin

s e l e c t s ta . s t a t i di n t o v s t a t i dfrom s ta tus s tawhere s ta . dsc = lower (new . ” s ta tus ” ) ;

except ionwhen no data found then

n u l l ; −− Need to ra i se an e r r o r ! ! ! !end ;


Insert Function II

−− f i n d i f there are any students w i th the same names e l e c t count ( s tu . s id )i n t o v nofrom student s tuwhere s tu . s name = new . ”name” ;i f v no > 0 then

n u l l ; −− r a i se an e r r o rend i f ;−− f i n d a new student i ds e l e c t coalesce (max( s tu . s id ) , 0 )i n t o v s i dfrom student s tu ;v s i d := v s i d + 1;i n s e r t i n t o s tudent

values ( v s id , new . ”name” , v s t a t i d ) ;r e t u r n new ;

end ;$$ language p lpgsq l ;


Insert Instead-Of-Trigger

Example (Create Trigger)

create t r i g g e r i n s s t u d e n t s t a t u sins tead of i n s e r t on s tu de n t s t a t usf o r each row execute procedure f i n s s t u d e n t s t a t u s ( ) ;

NoteAll inserts on student status will call stored procedureView is now updateable

Only insert supported

Very few limit on what can be done in stored procedure


Delete Functioncreate or rep lace f u n c t i o n f d e l s t u d e n t s t a t u s ( )

re tu rns t r i g g e r as $$dec lare

v s t a t i d s ta tus . s t a t i d%type ;begin

begins e l e c t s ta . s t a t i di n t o v s t a t i dfrom s ta tus s tawhere s ta . dsc = lower ( o ld . ” s ta tus ” ) ;

except ionwhen no data found then

n u l l ; −− Need to ra i se an e r r o r ! ! ! !end ;

de le te from studentwhere s name = old . ”name”and s t a t i d = v s t a t i d ;



Delete Instead-Of-Trigger

Example (Create Trigger)

create t r i g g e r d e l s t u d e n t s t a t u sins tead of de le te on s tu de n t s t a t usf o r each row execute procedure f d e l s t u d e n t s t a t u s ( ) ;

NoteAll delete on student status will call stored procedure

Similar idea as for the insert instead-of trigger

Trigger for update can be defined in a similar way


Use the Updateable View

Example (Update Through View)

−− i n s e r t through the viewi n s e r t i n t o s t ud en t s t a t usvalues ( ’ Curt ’ , ’ a c t i v e ’ ) ;

−− de le te through the viewde le te from s tu de n t s t a t us

where name = ’Ann ’ ;

NoteView now behave (almost) like a table


Summary: Instead-Of Triggers

SummaryA very powerful concept

Generally well-supported most DBMSs

Old or ugly design can removed and old application logic retained

NoteInsert, update, and delete can be combined in a single storedprocedure

Should make sense!

Too many triggers makes the database very hard to maintain!


Outline




4 Trigger Types


6 Managing Triggers

7 Summary


Enabling and Disabling Triggers

Example (Disable Triggers)−− d isab le s i n g l e t r i g g e ra l t e r t ab l e x d isab le t r i g g e r x upd a f te r row ;−− d isab le a l l t r i g g e r s on a s i n g l e t ab l ea l t e r t ab l e x d isab le t r i g g e r a l l ;

Example (Enable Triggers)−− enable s i n g l e t r i g g e ra l t e r t ab l e x enable t r i g g e r x upd a f te r row ;−− enable a l l t r i g g e r s on a s i n g l e tab lea l t e r t ab l e x enable t r i g g e r a l l ;

NoteBe careful, know why a trigger is disabled!












Trigger Metadata

Example (Find Information on All Triggers)s e l e c t ∗from informat ion schema . t r i g g e r s ;

NoteCannot see if trigger is enabled/disabled

Example (Find All Disabled Triggers)s e l e c t p .∗from p g t r i g g e r as pwhere p . tgenabled = ’D ’

NoteThis is PostgreSQL specific


Trigger Metadata

Example (Find Information on All Triggers)s e l e c t ∗from informat ion schema . t r i g g e r s ;

NoteCannot see if trigger is enabled/disabled

Example (Find All Disabled Triggers)s e l e c t p .∗from p g t r i g g e r as pwhere p . tgenabled = ’D ’

NoteThis is PostgreSQL specific


Dropping Triggers

Example (Find Information on All Triggers)−− Drop row t r i g g e r on tab l edrop t r i g g e r x upd a f te r row on x ;

−− Drop instead−of t r i g g e r on viewdrop t r i g g e r i n s s t u d e n t s t a t u s on s t ud en t s t a t us ;

NoteThe triggers are now removed from the database schema!

There is no syntax for dropping all triggers on a table!


Summary: Trigger Management

SummaryTriggers can be enabled/disabled

When table/view dropped all triggers automatically dropped

A view cannot always altered therefore triggers on views cannot bealtered

NoteDisabling triggers can lead to hard to find bugs

Many DBMSs have vendor specific extension for triggers


Outline




4 Trigger Types


6 Managing Triggers

7 Summary


Use or Avoid Triggers?

Use TriggersFor any type of logging

Also called auditing

For deriving information

For advanced checking that cannot be implemented using integrityconstraints

For automating tasks associated with modification statements

For simple data replicationTo wrap bad database design

That must be retained of backwards compatibility reasons

Avoid TriggersWhen a declarative statement can be used

Triggers are procedural

When foreign key or check statement can be used


Summary

E-C-A rules (event-condition-action)Triggers are called implicitly

Side-effects

{before, after} × {row, statement}

Instead-of triggers interesting for hiding legacy design Very powerfulconcept

Avoid transaction handling in triggers (commit or rollback)Do not overuse the number of triggers

Can make it very hard to find out what a system does

Many vendor specific extensions for trigger functionalityBecause very late before added to the standard

Core idea large overlap between DBMSs


Additional Information

PL/pgSQL www.postgres.cz/index.php/PL/pgSQL_(en)How to write the trigger stored procedures

Triggers in MySQL http://forge.mysql.com/wiki/TriggersGood overview, well explained, linear readable document

Exploring SQL Server Triggersmsdn.microsoft.com/en-us/magazine/cc164047.aspx

Good introduction, quite readable

Triggers in Oracle www.java2s.com/Tutorial/Oracle/0560__Trigger/Catalog0560__Trigger.htm

Only code, many details, Oracle specific

NoteCannot find a good general introduction to triggers

Send me an email if you are aware of such an introduction


www.postgres.cz/index.php/PL/pgSQL_(en)

http://forge.mysql.com/wiki/Triggers

msdn.microsoft.com/en-us/magazine/cc164047.aspx

www.java2s.com/Tutorial/Oracle/0560__Trigger/Catalog0560__Trigger.htm

www.java2s.com/Tutorial/Oracle/0560__Trigger/Catalog0560__Trigger.htm

Two-Minute Digesting

QuestionsWhy can you not specify the order in which triggers execute?

Why can triggers not be stand-alone database objects?

Why are triggers part of the transaction?

You have the same logic that must be executed by a trigger andsometimes explicitly what do you do?


Documents

Introduction to Triggers using SQL - AAUpeople.cs.aau.dk/~torp/Oracle/trigger.pdf · Learning Outcomes Goals Understand basic trigger mechanism Understand pros and cons of triggers