Triggers are database objects that are executed automatically in response to data modification statements like INSERT, UPDATE or DELETE on a table. There are different types of triggers - BEFORE triggers execute before the triggering statement while AFTER triggers execute after. Triggers can be used for data validation, integrity, auditing and more. Views can have INSTEAD OF triggers that allow modifications to views to be redirected to base tables.

1. Unit 4
Trigger
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2. Content:
 Triggers and their usage
 Trigger Activation
 BEFORE and AFTER Trigger
 INSTEAD OF Trigger
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3. 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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4. 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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5. 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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6. 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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7. 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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8. 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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9. 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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10. 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 ROW
BEGIN
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); 10

11. 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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12. 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);I
nsert 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_taken
referencing new as n
for each row mode db2sql
begin
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; 12

13. 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 row
BEGIN
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; 13

14. 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 ROW
begin
insert into auditwork values(r.empno,r.ename,r.job,current date);
end; 14

15. 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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16. 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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17. create table audit_log(
sid int,
sname varchar(20),
class varchar(10),
sdate date);
create or replace trigger del_student
after delete on student
referencing old as o
for each row
begin
insert into
audit_log(o.id,o.name,o.class,current date);
end; 17

18. 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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19. 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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20. 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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21. 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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22. 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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23. 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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24. 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_customer
instead of insert on customer_service
referencing new as n
for each row mode db2sql
begin
insert into customers values
(n.custno,n.custname,n.phone,n.credit_card,0);
end;
end
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