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Unit 4

Trigger

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Content:

Triggers and their usage

Trigger Activation

BEFORE and AFTER Trigger

INSTEAD OF Trigger

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Triggers

• Trigger : A trigger is a set of actions that will be executed

when defined event like insert, update and delete occurs.

• The trigger event can be following statement:

– Insert

– Update

– Delete

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Trigger

• Trigger is defined for specific table.

• Once trigger is defined, It will automatically active.

• A table have multiple triggers defined on it.

• If multiple triggers defined for a given table, the order of given

trigger activation is based on the trigger creation timestamp.

• Timestamp : order in which trigger were created.

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Trigger Storage

• Trigger definitions are stored in the system catalog table.

• Catalog View :

– SYSCAT.TRIGGERS

• Contains the trigger definition information, one row for

each trigger defined.

– SYSCAT.TRIDEP

• Contains one row for every dependency of trigger on

some other object.

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Trigger Activation

• Trigger can be defined to fire (be activate) in two way:

– Before Trigger

• Activated before integrity constraints are checked.

– After Trigger

• occur after the trigger event executes, and after the

database manager checks all constraints

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Syntax of Trigger

Create or replace trigger trig_name

Before|After insert | update | delete on table_name

Referencing new|old as var_name

for each row

Begin

….. Sql code…..

End

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Key Point :

Trigger Row trigger

Before Insert New

Before Update Old, New

Before Delete Old

After Insert New

After Update Old, New

After Delete Old

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Before Trigger

• A before trigger will fire for each row in the set of affected rows

before the triggering statement executes.

• Therefore, the trigger body is seeing the new data values prior

to their being inserted or updated into the table.

• A BEFORE trigger is activated before integrity constraints are

checked and may be violated by the trigger event.

• DML operations are not allowed in BEFORE triggers.

• code

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create table work(empno int not null primary key,ename varchar(10),job varchar(10),sal numeric(10,2),comm numeric(10,2));

CREATE OR REPLACE TRIGGER emp_comm_trig before INSERT ON work referencing new as n FOR EACH ROWBEGIN IF n.sal <= 2000 THEN set N.comm = N.sal * .4; else if N.sal <= 5000 THEN set N.comm = N.sal * .5; END IF;

end if;END;

insert into work values(101,'Jack','Salesman',3000,null);

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After Trigger

• An AFTER trigger occur after the trigger event executes, and

after the database manager checks all constraints that the

trigger event may affect, including actions of referential

constraints.

• Code

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create table test(test_id int not null primary key,name varchar(20),tdate date,tmarks int,pass_marks int);

insert into test values(1,'intrnal','01/01/2012',50,20);Insert into test values(2,'Quiz','05/02/2012',10,5);insert into test values(3,'Unit test','20/01/2012',25,10);

create table test_taken(cid int not null ,tid int references test,stime time,etime time,score int,pass_fail varchar(5));

create or replace trigger passfail after insert on test_takenreferencing new as nfor each row mode db2sqlbegin

declare pmark int;select pass_marks into pmark from test where test_id=n.tid;if(n.score>= pmark) then

UPDATE TEST_TAKEN SET PASS_FAIL='Pass' where cid=n.cid;else

UPDATE TEST_TAKEN SET PASS_FAIL='Fail' where cid=n.cid;end if;

end;

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CREATE TABLE empauditlog ( audit_date DATE, audit_user VARCHAR(20), audit_desc VARCHAR(20))

CREATE OR REPLACE TRIGGER emp_audit_trig AFTER INSERT OR UPDATE OR DELETE ON emp for each rowBEGIN v_action VARCHAR(20); IF INSERTING THEN set v_action := 'Added employee(s)'; ELSIF UPDATING THEN set v_action := 'Updated employee(s)'; ELSIF DELETING THEN set v_action := 'Deleted employee(s)'; END IF; INSERT INTO empauditlog VALUES (SYSDATE,USER,v_action);END;

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work(empno,ename,job,salary,commision)auditWork(id,name,job,sdate)

create table auditwork(id int,name varchar(15),job varchar(15),sdate date);

CREATE OR REPLACE TRIGGER del_trig After delete ON work referencing old as r FOR EACH ROWbegininsert into auditwork values(r.empno,r.ename,r.job,current date);end;

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Cascading Trigger

• Cascading triggers :Trigger can fire other trigger or same

trigger or other constraint are known as Cascading triggers.

• No Cascade is used to avoid cascading effects.

• No cascade is used after the trigger name.

Create or replace trigger check_id no cascade before insert on work

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Cont…

Create trigger check_time no cascade

before Insert on test_taken

Referencing new as n

For each row mode db2sql

begin

If (n.stime < ’08.30’) then

signal SQLSTATE ‘70003’

set message_text =‘can not start test before 8.30 am ‘;

end

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create table audit_log(sid int,sname varchar(20),class varchar(10),sdate date);

create or replace trigger del_studentafter delete on studentreferencing old as ofor each rowbegin

insert into audit_log(o.id,o.name,o.class,current date);

end;

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Trigger Usage• Data Validation :

– Ensure that a new data value is within the proper range.

• Data Conditioning :

– Implemented using triggers that fires before data record

modification.

• Data Integrity :

– Can be used to ensure cross-table dependencies are

maintained.

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Trigger Usage

• View Handling :

– Instead-of triggers allows the user to control how modifications

applied to view.

• Reduce amount of application development and make development

faster.

• Provide a global environment for your business rule.

– Defines once and stored in database, so available to all

application.

• Reduce maintenance of your application.

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View

•View : Data can be presented to the user in different logical

combinations, called views.

•View are used as a way of limiting access to base table.

•It can restrict both the columns and sets of rows that user can

access.

•code

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Con…

• To solve this problem, DBA can do following steps :

– Create a better database design to avoid this error.

– Allow null value in sales field or set default to zero.

– Create an instead of trigger.

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Instead of trigger

• Instead of trigger used only on views, not on base tables.

• It has similar characteristic to a normal trigger.

• Except for the following restriction.

– Only allow on view.

– Always for each row

– Default values get passed as null.

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Syntax :

Create or replace trigger trigger_name

instead of insert|update|delete on view_name

referencing new|old as row_variable

for each row mode db2sql

begin

……………..Sql statement………..

end;

code

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create table customers(custNo int not null,custName varchar(20) not null,phone char(12) not null,credit_Card varchar(20) not null,Sales decimal(15,2) not null);

Create view customer_service as( select custNo,custName,phone,credit_card from customers);

insert into customer_service values(1111,'John','888-999-0000','1111 2222 3333');

create or replace trigger det_customerinstead of insert on customer_servicereferencing new as nfor each row mode db2sqlbegin insert into customers values

(n.custno,n.custname,n.phone,n.credit_card,0);

end;

end