first attempt at learning oracle triggers - oracle

Question

I am just starting to learn triggers so please bear with me. If the row being inserted has a gift that is the same as any gift already in the table, print a message saying that the gift was already given to receiver from donor.
create or replace TRIGGER Same_Gift_Given
BEFORE INSERT ON GIVING
FOR EACH ROW
DECLARE
giftgiven varchar(255);
BEGIN
SELECT giftname INTO giftgiven from GIVING;
IF :new.giftname = giftgiven then
dbms_output.put_line(giftgiven || ' has already been gifted to ' || giving.receiver || ' by ' || giving.donor);
end if;
END;

Answer 1

This is a really awful homework problem. You would never, ever, ever us a trigger to do anything like this in a real system. It will break most INSERT operations and it will fail if there are ever multiple users. In reality, you would use a constraint. In reality, if for some reason you were forced at gunpoint to use a trigger, you would need a series of three triggers, a package, and a collection to do it properly.
What the professor is probably looking for
Just to emphasize, though, you would never, ever consider doing this in a real system
create or replace trigger same_gift_given
before insert on giving
for each row
declare
l_existing_row giving%rowtype;
begin
select *
into l_existing_row
from giving
where giftname = :new.giftname
and rownum = 1;
dbms_output.put_line( :new.giftname ||
' has already been gifted to ' ||
l_existing_row.receiver ||
' from ' ||
l_existing_row.donor );
exception
when no_data_found
then
null;
end;
This does not prevent you from inserting duplicate rows. It will throw a mutating trigger error if you try to do anything other than an INSERT ... VALUES on the giving table. It is inefficient. It does not handle multiple sessions. In short, it is absolutely atrocious code that should never be used in any real system.
What you would do in reality
In reality, you would create a constraint
ALTER TABLE giving
ADD CONSTRAINT unique_gift UNIQUE( giftname );
That will work in a multi-user environment. It will not throw a mutating trigger exception. It is much more efficient. It is much less code. It actually prevents duplicate rows from being inserted.

Answer 2

Let's try something a bit different:
CREATE OR REPLACE TRIGGER GIVING_COMPOUND_INSERT
FOR INSERT ON GIVING
COMPOUND TRIGGER
TYPE STRING_COL IS TABLE OF VARCHAR2(255) INDEX BY VARCHAR2(255);
colGiftnames STRING_COL;
aGiftname VARCHAR2(255);
nCount NUMBER;
-- Note that the way the associative array is used here is a bit of a cheat.
-- In the BEFORE EACH ROW block I'm putting the string of interest into the
-- collection as both the value *and* the index. Then, when iterating the
-- collection only the index is used - the value is never retrieved (but
-- since it's the same as the index, who cares?). I do this because I'd
-- rather not write code to call a constructor and maintain the collections
-- size - so I just use an associative array and let Oracle do the work for
-- me.
BEFORE EACH ROW IS
BEGIN
colGiftnames(:NEW.GIFTNAME) := :NEW.GIFTNAME;
END BEFORE EACH ROW;
AFTER STATEMENT IS
BEGIN
aGiftname := colGiftnames.FIRST;
WHILE aGiftname IS NOT NULL LOOP
SELECT COUNT(*)
INTO nCount
FROM GIVING
WHERE GIFTNAME = aGiftname;
IF nCount > 1 THEN
DBMS_OUTPUT.PUT_LINE('Found ' || nCount || ' instances of gift ''' ||
aGiftname || '''');
RAISE_APPLICATION_ERROR(-20001, 'Found ' || nCount ||
' instances of gift ''' ||
aGiftname || '''');
END IF;
aGiftname := colGiftnames.NEXT(aGiftname);
END LOOP;
END AFTER STATEMENT;
END GIVING_COMPOUND_INSERT;
Again, this is a LOUSY way to try to guarantee uniqueness. In practice the "right way" to do this is with a constraint (either UNIQUE or PRIMARY KEY). Just because you can do something doesn't mean you should.
Share and enjoy.