how to compare the values inside a table in sql

Question

how to compare the values of same table(say for eg: Order table) each and every time the record get inserted , if the record with same values get inserted already in same table i should not insert the new record with same values. how to do that exactly in sql server 2008

Answer 1

If exists(select * from Order where key_column=@some_value)
print 'data already exists'
else
Insert into Order(columns) values (@some_value,...)

Answer 2

I'd suggest adding a unique index on the key columns...

ALTER TABLE mytable ADD UNIQUE INDEX myindex (keycolumn1, keycolumn2, ...);

That'd make it impossible to insert a duplicate by accident.