what is the use of Trigger

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Here are some very good articles on Trigger.
Check this:
Triggers -- SQL Server[^]
http://www.sqlteam.com/article/an-introduction-to-triggers-part-i[^]
http://msdn.microsoft.com/en-us/library/ms189799.aspx[^]

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trigger fires when insert,Update ,delete statemet execute


eg:

if we have table name : t1
if we calculate the sum and updated in another table t2 when insert into t1 occurs,


eg:

CREATE OR REPLACE TRIGGER tr1
BEFORE
insert on sampletable
for each row
begin
SELECT sampletable2.id.nextval INTO :NEW.R_ID  FROM dual;
end;

cursor:

What is Cursor in sql server[^]