For an assignment I need to write an Oracle SQL trigger that prevents the StdBalance column in the STUDENT table from exceeding 500. Normally I would just use a check constraint to enforce this however I am being forced to write a trigger for it. As expected Oracle is throwing a mutating error due to me using SELECT in an update trigger and will not let me update the StdBalance value with anything. Does anyone have any idea how I could work around this? Here is the code for the trigger.
CREATE OR REPLACE TRIGGER Balance_Trigger
BEFORE UPDATE ON STUDENT
FOR EACH ROW
DECLARE
Current_Balance NUMBER;
BEGIN
SELECT :new.StdBalance
INTO Current_Balance
FROM STUDENT
WHERE :new.stdID = StdID;
IF Current_Balance > 500
THEN Raise_Application_error(-20007, 'You cannot exceed an unpaid balance of $500');
end if;
end;
/
show error;
Just use :
Current_Balance := :new.StdBalance;
Instead of
SELECT :new.StdBalance
INTO Current_Balance
FROM STUDENT
WHERE :new.stdID = StdID;
to suppress the mutating error.
P.S. Even if such assignments are used in these cases, as David Faber warned, there's no need to return a value for Current_Balance, :new.StdBalance may be used directly in comparison for IF :new.StdBalance > 500 ....
Related
I'm trying to create a trigger to validate if a new entry in the table registraties (registrations) contains a valid MNR (employee number) but I'm getting stuck on the part where I'm referencing the table medewerkers (employees).
Could someone help me out?
CREATE OR REPLACE TRIGGER t_MNRcontrole
BEFORE INSERT OR UPDATE
ON registraties
DECLARE
MNR_medewerkers number (SELECT MNR FROM MEDEWERKERS);
FOR EACH ROW
BEGIN
IF :new.MNR <> MNR_medewerkers
THEN raise_application_error(-20111, 'Medewerker niet herkend!');
END IF;
END;
Error message received is
ORA-24344: success with compilation error
The PL/SQL assignment operator is :=, or select x into y from z to populate from a SQL query.
FOR EACH ROW is part of the trigger spec, not the PL/SQL code.
If :new.mnr is not present in the parent table, you will get a no_data_found exception, not a mismatched variable.
It's good practice for error messages to include details of what failed.
In programming, we use indentation to indicate code structure.
A fixed version would be something like:
create or replace trigger trg_mnrcontrole
before insert or update on registraties
for each row
declare
mnr_medewerkers medewerkers.mnr%type;
begin
select mw.mnr into mnr_medewerkers
from medewerkers mw
where mw.mnr = :new.mnr;
exception
when no_data_found then
raise_application_error(-20111, 'Medewerker '||:new.mnr||' niet herkend!');
end;
However, we can implement this kind of check better using a foreign key constraint, for example:
alter table registraties add constraint registraties_mw_fk
foreign key (mnr) references medewerkers.mnr;
MNR_medewerkers number (SELECT MNR FROM MEDEWERKERS);
will always fail because its not a NUMBER, unless your table happens to only have one single entry and even then I am not sure PLSQL will allow it to pass.
The more standard case for this would be to first declare the number, then in the codeblock you do a SELECT INTO along with a WHERE clause where you make sure to only pick one specific row from the table. Then you can compare that number with the new one.
If however you are not trying to compare to one specific row, but are instead checking if the entry exists in that table.
BEGIN
SELECT 1
INTO m_variable
FROM table
WHERE MNR = :new.MNR;
EXCEPTION
WHEN TOO_MANY_ROWS THEN
m_variable = 1;
WHEN OTHERS THEN
m_variable = 0;
END;
Declare the m_variable beforehand, and then check if its 0 then report the error.
The too_many_rows is in case there is more than one row in the table with this MNR, and the OTHERS is there for the NO_DATA_FOUND, but I use OTHERS to handle everything else that could happen but probably wont.
Btw this is a code block to be included within the main code block, so between your BEGIN and IF, then just change the IF to check if the variable is 0.
How do I write a simple trigger to check when salary is updated it is not over 5% for an existing faculty member.
I have a block of code here.
create or replace TRIGGER TRG_Not_over_5per
BEFORE UPDATE OF F_SALARY ON FACULTY
DECLARE
sal FACULTY.F_SALARY%TYPE;
BEGIN
SELECT FACULTY.F_SALARY INTO sal FROM FACULTY
-- how do I use the WHERE CLAUSE here so that I get the salary affected
-- do some math on the first and second salary
-- if less than 5%, keep update, if not, reject update.
END;
Thanks in advance.
You don't need to use a SELECT in the trigger (*). Define the trigger as FOR EACH ROW and you can have access to the old and new values of any column belonging to the table.
create or replace TRIGGER TRG_Not_over_5per
BEFORE UPDATE OF F_SALARY ON FACULTY
FOR EACH ROW
So your code would look like:
if :new.f_salary < :old.f_salary * 1.05 then
raise_application_error (
-20000
, 'salary increase must be at least 5%'
);
end if;
This way of handling the rule violation is just a suggestion. You do whatever you need to. You don't need to handle the ELSE branch: Oracle will apply the update by default.
(*) In fact Oracle will hurl a mutating table exception if you do try to execute the query you want to write. Find out more.
I am trying to make a Library Infotainment System using PL/SQL. Before any of you speculate, yes it is a homework assignment but I've tried hard and asking a question here only after trying hard enough.
Basically, I have few tables, two of which are:
Issue(Bookid, borrowerid, issuedate, returndate) and
Borrower(borrowerid, name, status).
The status in Borrower table can be either 'student' or 'faculty'. I have to implement a restriction using trigger, that per student, I can issue only 2 books at any point of time and per faculty, 3 books at any point of time.
I am totally new to PL/SQL. It might be easy, and I have an idea of how to do it. This is the best I could do. Please help me in finding design/compiler errors.
CREATE OR REPLACE TRIGGER trg_maxbooks
AFTER INSERT ON ISSUE
FOR EACH ROW
DECLARE
BORROWERCOUNT INTEGER;
SORF VARCHAR2(20);
BEGIN
SELECT COUNT(*) INTO BORROWERCOUNT
FROM ISSUE
WHERE BORROWER_ID = :NEW.BORROWER_ID;
SELECT STATUS INTO SORF
FROM BORROWER
WHERE BORROWER_ID = :NEW.BORROWER_ID;
IF ((BORROWERCOUNT=2 AND SORF='STUDENT')
OR (BORROWERCOUNT=3 AND SORF='FACULTY')) THEN
ROLLBACK TRANSACTION;
END IF;
END;
Try something like this:
CREATE OR REPLACE TRIGGER TRG_MAXBOOKS
BEFORE INSERT
ON ISSUE
FOR EACH ROW
BEGIN
IF ( :NEW.BORROWERCOUNT > 2
AND :NEW.SORF = 'STUDENT' )
OR ( :NEW.BORROWERCOUNT > 3
AND :NEW.SORF = 'FACULTY' )
THEN
RAISE_APPLICATION_ERROR (
-20001,
'Cannot issue beyond the limit, retry as per the limit' );
END IF;
END;
/
There should not be a commit or rollback inside a trigger. The logical exception is equivalent to ROLLBACK
This is so ugly I can't believe you're being asked to do something like this. Triggers are one of the worst ways to implement business logic. They will often fail utterly when confronted with more than one user. They are also hard to debug because they have hard-to-anticipate side effects.
In your example for instance what happens if two people insert at the same time? (hint: they won't see the each other's modification until they both commit, nice way to generate corrupt data :)
Furthermore, as you are probably aware, you can't reference other rows of a table inside a row level trigger (this will raise a mutating error).
That being said, in your case you could use an extra column in Borrower to record the number of books being borrowed. You'll have to make sure that the trigger correctly updates this value. This will also take care of the multi-user problem since as you know only one session can update a single row at the same time. So only one person could update a borrower's count at the same time.
This should help you with the insert trigger (you'll also need a delete trigger and to be on the safe side an update trigger in case someone updates Issue.borrowerid):
CREATE OR REPLACE TRIGGER issue_borrower_trg
AFTER INSERT ON issue
FOR EACH ROW
DECLARE
l_total_borrowed NUMBER;
l_status borrower.status%type;
BEGIN
SELECT nvl(total_borrowed, 0) + 1, status
INTO l_total_borrowed, l_status
FROM borrower
WHERE borrower_id = :new.borrower_id
FOR UPDATE;
-- business rule
IF l_status = 'student' and l_total_borrowed >= 3
/* OR faculty */ THEN
raise_application_error(-20001, 'limit reached!');
END IF;
UPDATE borrower
SET total_borrowed = l_total_borrowed
WHERE borrower_id = :new.borrower_id;
END;
Update: the above approach won't even work in your case because you record the issue date/return date in the issue table so the number of books borrowed is not a constant over time. In that case I would go with a table-level POST-DML trigger. After each DML verify that every row in the table validates your business rules (it won't scale nicely though, for a solution that scales, see this post by Tom Kyte).
I need to create a TRIGGER that sets a flag when a product’s quantity on hand falls below 5.
I've created a table "STP_STOCK" in Oracle 11g and it has the following column:
STP_QUANTITY, NUMBER(4,0), RANGE 0-9999
How do I create a trigger that sets a flag in SQL?
I've got as far as the following... but have no idea re setting a flag.
CREATE OR REPLACE TRIGGER STP_STOCK
AFTER UPDATE
ON orders
FOR STP_QUANTITY
BEGIN
END;
Assuming the flag is in the same table, I would suggest you use a before update trigger. Then you can do something like:
CREATE OR REPLACE TRIGGER STP_STOCK
before UPDATE
ON orders
FOR each row
BEGIN
if :new.stp_quantity < 5
then :new.flag := 'Y';
end if;
END;
(I don't have Oracle on hand to test the syntax, so the syntax might have an error.)
You want to do a before trigger, so you can just set the value in :new. It is bad practice to update the same table in an after trigger.
Presumably, you also want to check if the quantity is larger than 5 and set the flag to 'N', but that is not specified in the question.
Finally, instead of a trigger and a flag, you probably really want to do this with a view or virtual column. Something like:
select o.*, (case when stp_quantity < 5 then 'Y' else 'N' end) as Flag
from orders o
(Note: I'm using * as a convenience. In practice, you would want to list each column in the view.)
I have account table as this--
create table account
(
acct_id int,
cust_id int,
cust_name varchar(20)
)
insert into account values(1,20,'Mark');
insert into account values(2,23,'Tom');
insert into account values(3,24,'Jim');
I want to create a trigger which will ensure that no records can be inserted or update in account table having acct_id as 2 and cust_id as 23.
My code is --
create trigger tri_account
before insert or update
on account
for each row
begin
IF (:new.acct_id == 2 and :new.cust_id == 23) THEN
DBMS_OUTPUT.PUT_LINE('No insertion with id 2 and 23.');
rollback;
END IF;
end;
so this trigger is created , but with compilation error.
now when I insert any record with acct_id as 2 and cust_id as 23,it doesent allow.
But I get an error saying
ORA-04098: trigger 'OPS$0924769.TRI_ACCOUNT' is invalid and failed re-validation
I don't understand this.I also want to show a message that dis insertion is not possible.
please Help...
The equality operator in Oracle is =, not ==.
You cannot commit or rollback in a trigger. You can throw an exception which causes the triggering statement to fail and to be rolled back (though the existing transaction will not necessarily be rolled back).
It does not appear that this trigger compiled successfully when you created it. If you are using SQL*Plus, you can type show errors after creating a PL/SQL object to see the compilation errors.
You should never write code that depends on the caller being able to see the output from DBMS_OUTPUT. Most applications will not so most applications would have no idea that the DML operation failed if your trigger simply tries to write to the DBMS_OUTPUT buffer.
Putting those items together, you can write something like
create trigger tri_account
before insert or update
on account
for each row
begin
IF (:new.acct_id = 2 and :new.cust_id = 23) THEN
raise_application_error( -20001, 'No insertion with id 2 and 23.');
END IF;
end;
A trigger is more flexible, but you can also accomplish this through the use of a CHECK CONSTRAINT:
ALTER TABLE account ADD CONSTRAINT check_account CHECK ( acct_id != 2 OR cust_id != 23 )
ENABLE NONVALIDATE;
The NONVALIDATE clause will ensure that the check constraint does not attempt to validate existing data, though it will validate all future data.
Hope this helps.
IF (:new.acct_id = 2 and :new.cust_id = 23) THEN
must be OR, not and.
While using conditional checks you don't need to use colons (:). This will always cause errors.
Note: Exclude the colon only in cases where condition checking is performed.