This chapter describes triggers, their types, creation and dropping of the triggers.
Introduction
A trigger is a set of actions, which are performed for responding to an INSERT, UPDATE or DELETE operation on a specified table in the database. Triggers are stored in the database at once. They handle governance of data. They can be accessed and shared among multiple applications. The advantage of using triggers is, if any change needs to be done in the application, it is done at the trigger; instead of changing each application that is accessing the trigger. Triggers are easy to maintain and they enforce faster application development. Triggers are defined using an SQL statement “CREATE TRIGGER”.
Types of triggers
There are two types of triggers:
1. BEFORE triggers
They are executed before any SQL operation.
2. AFTER triggers
They are executed after any SQL operation.
Creating a BEFORE trigger
Let us see how to create a sequence of trigger:
Syntax:
db2 create sequence <seq_name>
Example: Creating a sequence of triggers for table shopper.sales1
db2 create sequence sales1_seq as int start with 1 increment by 1
Syntax:
db2 create trigger <trigger_name> no cascade before insert on <table_name> referencing new as <table_object> for each row set <table_object>.<col_name>=nextval for <sequence_name>
Example: Creating trigger for shopper.sales1 table to insert primary key numbers automatically
db2 create trigger sales1_trigger no cascade before insert on shopper.sales1 referencing new as obj for each row set obj.id=nextval for sales1_seq
Now try inserting any values:
db2 insert into shopper.sales1(itemname, qty, price)
values('bicks', 100, 24.00)
Retrieving values from table
Let us see how to retrieve values from a table:
Syntax:
db2 select * from <tablename>
Example:
db2 select * from shopper.sales1
Output:
ID ITEMNAME QTY
------- ------------ ----------
3 bicks 100
2 bread 100
2 record(s) selected.
Creating an AFTER trigger
Let us see how to create an after trigger:
Syntax:
db2 create trigger <trigger_name> no cascade before insert on <table_name> referencing new as <table_object> for each row set <table_object>.<col_name>=nextval for <sequence_name>
Example: [To insert and retrieve the values]
db2 create trigger sales1_tri_after after insert on shopper.sales1 for each row mode db2sql begin atomic update shopper.sales1 set price=qty*price; end
Output:
//inseting values in shopper.sales1
db2 insert into shopper.sales1(itemname,qty,price)
values('chiken',100,124.00)
//output
ID ITEMNAME QTY PRICE
----- -------------- ----------- -----------
3 bicks 100 2400.00
4 chiken 100 12400.00
2 bread 100 2400.00
3 record(s) selected.
Dropping a trigger
Here is how a database trigger is dropped:
Syntax:
db2 drop trigger <trigger_name>
Example:
db2 drop trigger slaes1_trigger