By Muhammad Fahim Khan

8/14/2019 By Muhammad Fahim Khan

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By Muhammad Fahim Khan


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Objectives

Trigger ConceptTrigger RequirementTrigger - UsesTrigger Types

Referencing columns :OLD Vs :NewTrigger Important points

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Triggers Concept

Triggers are special PL/SQL construct similar to procedures.

However, a procedure is executed explicitly from another block via a procedure call.

A trigger is executed implicitly or Automatically

whenever the triggering event happens.

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Triggers Concept

Triggers are basically created and stored likestored procedure in database.

Once they are stored then they are automaticallycompiled and called by database itself, under a

given condition or what we call as TriggeringEvent.

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Triggers Why

Triggers provides support for followingactivities in database:

Validation : There are certain validations on tableand its data that we cannot provide using constraintsso for all such cases we use triggers for validations.

Eg:In a bank if we want to provide some validation for withdrawal of a money in transaction table bychecking the balance in the Account master table.

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Triggers Why

Auditing : Triggers are automatically called whensome action happens on a table, so they can be usedto keep track for all the activities happening on a

table.

Eg:Triggers can be written on a table emp, dept etc tocheck and store the name of the person , date andtime of any operation and type of operationhappening on these tables in some other audit table .

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Triggers Why

Backup : Triggers can be used to automatically back up some crucial data.

Eg:Trigger can be written on Emp table to store theempno , name and salary for future use in some back table, if somebody deletes the record in the emp.

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Triggers Why

Restrictions: Triggers can be used to imposerestriction or prohibit some operation under certainconditions.

Eg:Trigger can be written on Emp table to disallow or

prohibit anybody from deleting or modifying any

record in Emp table on Sat and Sun.

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Triggers Types

DML Triggers : Triggers on INSERT ,UPDATE , DELETE .

DML triggers are used by developers for validations , auditing , backup etc.

DML triggers are written on tables

Requires no special privileges.

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Triggers Types

System level triggers : Database level Trigger : Triggers on startup and shutdown of

database.

Schema level Trigger :

Triggers on Logon and Logoff from schema. DDL Trigger : Triggers on CREATE , ALTER , DROP

System level trigger are used by Database Administrator (DBA)for monitoring and administration purposes

Requires special privileges

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Triggers

Note :

In current session we will deal with DMLtriggers only as they are used for tablelevel operations. Other trigger requires

4. Special privileges (DBA)5. Not in the scope/expectation of the current

session

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Triggers Syntax

CREATE [OR REPLACE] TRIGGER

{TRIGGERING_TIME }

{TRIGGERING_EVENT } ON < TABLE_NAME >

[FOR EACH ROW [WHEN (< TRIGGER_CONDITION >)] ]

DECLARE

BEGIN

EXCEPTION

END;

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Triggers Syntax

TRIGGERING_TIME Before : Specify before if database should

fire trigger (execute trigger) before triggeringevent.

After :Specify After if database should firetrigger (execute trigger) after triggering event.

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Triggers Syntax

TRIGGERING_EVENT INSERT : If trigger should be fired or executed for Insert

operation.

UPDATE : If trigger should be fired or executed for updateoperation.

DELETE : If trigger should be fired or executed for Deleteoperation.

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Triggers Syntax

TABLE_NAME Name of the table for which trigger should execute.

Remember one DML trigger is always associated withone table i.e. a same DML trigger cannot be written for two different tables.

Each trigger has a unique name.

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Triggers Syntax

NOTE:For practice of trigger examples.

Please make your own copy of table and

give different name for triggers preferablywith your own employee no else you willover ride others persons triggers or two same

type of triggers on same table will causeambiguity and errors.

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Triggers Syntax

FOR EACH ROW [WHEN (< TRIGGER_CONDITION >)]

DML triggers can be Row level : Executes one time for each row.

Statement level : Executes one time for each statement.

For each row is optional , if you specify it then trigger is rowlevel trigger. When and condition should not be given here.

If we do not specify for each row then it is statement level ,you can give some condition with WHEN for the statementlevel trigger , it can be some query etc but not a sub-query

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Triggers SyntaxROW Level Vs Statement Level Example : Suppose there is a Before Update trigger on emp table.

So if we give a update query on SQL prompt then trigger will beexecuted automatically by oracle.

Update Emp Set Sal = Sal + 500 ;

Query is nothing but its like a statement and this update query canupdate many rows in a table Emp here.

If we want trigger to execute only once before update for all thoserows then we will use statement level i.e. without for each row .

If we want trigger to execute once before every row the abovestatement or query will update then row level trigger i.e. with for each row .

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Triggers Syntax

Mostly Row level is used for DML Triggers.

For Validation , Backup , Auditing , Restriction etcrow level can be used.

Statement level mostly is used for Auditing purposes

only.

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Triggers Syntax

DECLARE : Declaring local variables for the trigger.

BEGIN : Contains the actual code for the trigger toexecute whenever triggering event happens.

EXCEPTION: Exception or Error handling code.

Basically these are same as normal block of PL SQL

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Trigger Important

Transaction control statements like Commit , Rollback etc are not allowed in trigger.

Create , Alter and Drop (DDL) statements are notallowed in trigger.

Insert , Update and Delete (DML) inside the trigger code is allowed but only on tables other then the tableon which trigger itself is written.

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Trigger Important

Select query inside the trigger code on all tables isallowed but query on the table on which trigger iswritten can cause some problems , precautions should

be taken.

Calling procedure , functions and packages insidetrigger is allowed but those procedures , functions and

packages should not contain any commit or rollback statements.

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Trigger Important

Mutating Table : "Mutating" means "changing".A mutating table is a table that is currently beingmodified by an update, delete, or insert statement. Whena trigger tries to refer a table that is in state of flux (beingchanged), it is considered "mutating" and raises an error

Another way this error can occur is if the trigger hasstatements to change the primary, foreign or unique key

columns of the table off which it fires. If we write somecode in trigger that can cause ambiguity with that table :

Example If we write a Insert query on the table in aInsert trigger of that table itself which is not allowed then

at runtime it will give Mutation error .

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Trigger Important

To avoid mutation problem avoid using Insert ,update , Delete or Select query on a mutating

table in the trigger code i.e. the same table onwhich the trigger is written.

However you can write Insert , Update andDelete query on other tables in the trigger code.

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:OLD and :NEW

Inside the trigger code since we cannot refer to thetable on which trigger is written directly so if we wantto check or change or verify the value in some columnof that table we can used :OLD and : NEW.

:OLD and :NEW is only used in row level triggers.

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:OLD and :NEW

:OLD and :NEW refers to the row of the table onwhich trigger is currently executing.

Any column of that row can be referred to as:OLD.Column_Name or :NEW.Column_Name

Example: OLD.Empno , :OLD.Ename ,

:NEW.Empno etc

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:OLD and :NEW

:OLD Vs :NEW

In case of INSERT new data comes to table so we can refer to thecolumns of that new row of the table in the trigger with :NEW.

:OLD is invalid in Insert triggers.

In case of DELETE old data is deleted from table so we can refer to the columns of that old row of the table being deleted in thetrigger with :OLD.

:NEW is invalid in Delete triggers.

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:OLD and :NEW

:OLD Vs :NEW

In case of UPDATE old data is modified with newdata so we can refer to the columns of that row of thetable in the trigger with both :OLD and :NEW.

:OLD will point to old data of the row.

:New will point to new data being updated for thatrow.

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or Invalid:OLD.Col_Name

DELETE

:NEW.Col_Name:OLD.Col_NameUDATE

:NEW.Col_Name or InvalidINSERT

:NEW:OLD

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:OLD and :NEW

Example : In dept table there is one record

Deptno Dname Loc10 Admin Pune

Suppose there is a update trigger on this table then wecan have both :OLD and : NEW in that trigger.

Suppose we give the update query as :Update Dept Set Loc = Mumbai where Deptno = 10;

Cont.

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:OLD and :NEW

Inside the trigger then

:OLD.Deptno 10

:OLD.Dname Admin

:OLD.Loc Pune

:NEW.Deptno 10:NEW.Dname Admin

:NEW.Loc Mumbai

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Trigger Important

One table we can have all types of trigger each with unique name i.e.Emp can have 12 triggers Before Update Statement level

Before Update Row level After Update Row level

After Update Statement level

Before Insert Statement level Before Insert Row level

After Insert Row level

After Insert Statement level

Before Delete Statement level

Before Delete Row level

After Delete Row level

After Delete Statement level

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Trigger Important

Suppose if we have four trigger on emp table

Before Update Statement level

Before Update Row level After Update Row level

After Update Statement level

Cont.

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Trigger Important

Update Emp set Sal = Sal + 500 where Deptno = 10;Suppose this query affects 3 rows.

Trigger will be executed as follows and in followingorder.

Before Update Statement level (Only Once) Before Update Row level After Update Row level

Before Update Row level After Update Row level Before Update Row level After Update Row level After Update Statement level (Only Once)

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Trigger Important

Row level is executed one time each for each row.

Statement level only once for one query.

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Trigger Important

However on same table you cannot haveexactly two same type of triggers , they cancause problems i.e. Two triggers on emp both

before update row level ,can cause errors.

As table will not understand which one toexecute for each row before update happens.

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Trigger Important

Triggers are one of the important andvery excellent feature of Oracle PL SQL

but should be used properly as it slowsdown the speed of DML operations.

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ExampleCreate following tables for examples Tables.SQL

Example 51 Sample51.SQL






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http://tables.sql/http://sample51.sql/http://sample52.sql/http://sample53.sql/http://sample54.sql/http://sample55.sql/http://sample56.sql/http://sample56.sql/http://sample55.sql/http://sample54.sql/http://sample53.sql/http://sample52.sql/http://sample51.sql/http://tables.sql/


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USER_TRIGGERS

To see all the triggers created you can view USER_TRIGGERS

To see the structure Desc USER_Triggers

To see triggers for EMP_ESG table

select trigger_type, triggering_event,table_name, from user_triggers wheretable_name = EMP_ESG';


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Deleting TRIGGERS

To delete trigger

Drop trigger ;

Example:

Drop Trigger Emp_Up_1;

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Next Session:

Object types inOracle

Thank you !!Thank you !!

By Muhammad Fahim Khan

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