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
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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