SELECT CASE to use NOT EXISTS in Oracle failing - oracle

Question

I have been trying to find a solution to use an If_Exists() style statement in Oracle PL SQL. I am trying to create a trigger which checks to see if a certain airsoft gun exists in the guns table when a member tries to input a new gun owned in the gunsOwned table. If the gun does not exist in the guns table, then it must be inputted to the table before the gun owned is inputted to the gunsOwned table or it will violate referential integrity as the Make and Model in gunsOwned are foreign keys to the Make and Model in the Guns table. However I keep getting Trigger created with compilation errors, and all of my attribute names are correct, so don't know why the select case statement is not working. Here is the code:
CREATE TRIGGER updateGuns
BEFORE INSERT ON GunsOwned
FOR EACH ROW
DECLARE
MemberAddingGun NUMBER;
NewMake VARCHAR2(30);
NewModel VARCHAR2(30);
BEGIN
MemberAddingGun := :NEW.OwnerID;
NewMake := :NEW.MakeOwned;
NewModel := :NEW.ModelOwned;
SELECT CASE gunExists
WHEN NOT EXISTS(SELECT Make, Model FROM Guns WHERE Make=NewMake AND Model=NewModel)
THEN
INSERT INTO Guns VALUES(NewMake, NewModel);
END
UPDATE Member
SET NumOfGuns = NumOfGuns+1
WHERE MemberID = MemberAddingGun;
END updateGuns;
.
RUN;
Could anyone help?
Thanks!

Answer 1

Use simple INSERT ... SELECT ... WHERE instead of CASE or IF statements:
INSERT INTO Guns( colname1, colname2 )
SELECT NewMake, NewModel FROM dual
WHERE NOT EXISTS(
SELECT null FROM Guns WHERE Make=NewMake AND Model=NewModel
);
BTW - on multiuser environment checking for not-existence of a record will always fail, since not commited records are not visible to SQL, and you will get duplicate records in Guns table.
In such a case you need some kind of synchronization.

Answer 2

There are a couple of options. First, you can handle this using a MERGE statement:
CREATE TRIGGER updateGuns
BEFORE INSERT ON GunsOwned
FOR EACH ROW
BEGIN
MERGE INTO GUNS
USING (SELECT MAKE, MODEL FROM GUNS) g
ON (g.MAKE = :NEW.MAKEOWNED AND g.MODEL = :NEW.MODELOWNED)
WHEN NOT MATCHED THEN
INSERT (MAKE, MODEL)
VALUES (:NEW.MAKEOWNED, :NEW.MODELOWNED);
UPDATE Member
SET NumOfGuns = NumOfGuns+1
WHERE MemberID = :NEW.OWNERID;
END UPDATEGUNS;
In this case the MERGE acts as a conditional INSERT, only adding a new row to GUNS if the specified make and model don't already exist in the table.
Alternatively, assuming that MAKE and MODEL are either the primary key or are a unique key on GUNS you can just go ahead and do the INSERT, trap the DUP_VAL_ON_INDEX exception thrown if a duplicate is found, and proceed merrily on your way:
CREATE TRIGGER updateGuns
BEFORE INSERT ON GunsOwned
FOR EACH ROW
BEGIN
BEGIN
INSERT INTO GUNS
(MAKE, MODEL)
VALUES
VALUES (:NEW.MAKEOWNED, :NEW.MODELOWNED);
EXCEPTION
WHEN DUP_VAL_ON_INDEX THEN
NULL; -- ignore the DUP_VAL_ON_INDEX exception
END;
UPDATE Member
SET NumOfGuns = NumOfGuns+1
WHERE MemberID = :NEW.OWNERID;
END UPDATEGUNS;
Personally, I don't like ignoring exceptions - I'd rather write code which doesn't raise exceptions - but it's your choice.
Best of luck.

Answer 3

Just use IF after setting up an appropriate flag:
DECLARE
v_flag number;
BEGIN
SELECT (CASE WHEN EXISTS (SELECT 1
FROM Guns
WHERE Make = :New.MakeOwned AND Model = :New.Model AND rownum = 1;
)
THEN 1 ELSE 0
END)
INTO v_flag
FROM DUAL;
IF v_flag = 0
THEN
INSERT INTO Guns(Make, Model) VALUES (:New.Make, :New.Model);
END IF;
UPDATE Member
SET NumOfGuns = NumOfGuns + 1
WHERE MemberID = :New.OwnerId;
END; -- updateGuns
I see no advantage to copying the fields in :NEW to local variables. In fact, it makes the code a bit harder to follow, because the reader has to check if the values are different from the values in the :NEW record.
That said, an alternative is to have a unique index on Guns(Make, Model), attempt an insert and just ignore the error using exceptions.