I'm trying to make a condition where if the number of available seats within a given plane is less than zero then ROLLBACK. However, I am receiving a message stating that there is a syntax error at IF (i.e. "syntax error at or near "IF""). Why is this, can I not use if statements in transactions? And if so, how can I perform this conditional statement?
BEGIN;
UPDATE FlightBooking SET NumSeats = NumSeats + 100, TotalCost = TotalCost + 100
WHERE CustomerID = 20005;
IF (SELECT Available_Seats FROM checkAvailability(30001) < 0) THEN
ROLLBACK;
END IF;
COMMIT;
Many thanks, callum
Use a check constraint!
alter table flightbooking add constraint chk_flightbooking_availability
check (availability >= 0);
That way, the update simply fails and the data never fails to meet your condition.
Alas, if you want to use your function, you need to convert it to a scalar value:
alter table flightbooking add constraint chk_flightbooking_availability
check (checkAvailability_scalar(30001) >= 0);
That said, it is probably better to store that value somewhere and use a check constraint on that table.
Personally I would do the check before the update... that way you don't have to roll back.
Here is how to fix your if statement.
IF (SELECT Available_Seats FROM checkAvailability(30001)) < 0 THEN
ROLLBACK;
END IF;
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.
is it possible to use the insert value in the where clause in SQL?
I would want to use it in this kinda way:
create trigger t_wage_not_higher_than_30000
on transaction
as
if exists
(
select *
from transaction
where ***inserted value*** >= 30000 and
description = employeewage
)
begin
raiserror('you cannot insert a wage higher than 30000')
rollback transaction
end
If you want to check the range of values, the best way is to use a check constraint:
alter table transactions add constraint chk_transactions_value
check (value < 30000);
There is no reason to write a trigger for checking data values.
For this trigger, you should only refer to the newly inserted rows. Therefore, you need to use the special trigger table in your statement. E.g.,
if exists (select * from inserted
where <some column you did not name in
your code> >= 30000 and description = 'employeewage')
begin
raiserror ... (or throw);
rollback tran;
return;
end;
And yes - a constraint is generally a better approach.
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 ....
CREATE TRIGGER Alerta_Trig
BEFORE INSERT OR UPDATE OF Valor ON Medicao
FOR EACH ROW
WHEN (NEW.VALOR > 40)
BEGIN
UPDATE SENSOR
SET ALERTA = 'Alerta laranja'
WHERE SENSOR_ID=(SELECT SENSOR_SENSOR_ID FROM MEDICAO);
END;
I have this table of Sensors that are supposed to receive a String with an alert if 1 value > 40 enters the Medicao table. Any thoughs?
As you haven't said quite what is wrong I'm speculating a bit, but I can see two immediate potential problems.
You shouldn't query the same table again, generally, as you'll risk get a mutating table error. You also have no condition in the subquery so you'd be likely to get multiple rows back, and a subquery-returns-more-than-one-row error.
Presumably you're doing that subquery to try to get the ID value from the affected row, so use the :NEW pseudo-row for that:
CREATE TRIGGER Alerta_Trig
BEFORE INSERT OR UPDATE OF Valor ON Medicao
FOR EACH ROW
WHEN (NEW.VALOR > 40)
BEGIN
UPDATE SENSOR
SET ALERTA = 'Alerta laranja'
WHERE SENSOR_ID = :NEW.SENSOR_SENSOR_ID;
END;
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.