Creating a Trigger that runs on two tables - sql

Question

I have two tables, COPY and BORROW. Their columns are:
COPY (
Copy_id,
Bk_id,
Loc_id,
Opinion
)
and
BORROW (
Cus_evo,
B_Date,
R_Date,
Fee,
Copy_id,
Cus_id
)
I want to configure a trigger that ensures Copies that stored in a Exact location (London) (from Loc-id column of COPY table) cannot be Borrowed in December (from B_Date column of BORROW table).
I have created the following trigger:
CREATE OR REPLACE TRIGGER BORROW_TRIGGER
BEFORE INSERT ON BORROW FOR EACH ROW BEGIN
IF(TO_CHAR(TO_DATE(:NEW.B_Date, 'DD-MMM-YYYY'),'MMM'= 'DEC')
AND :NEW.Loc_id='LC0001')
THEN RAISE_APPLICATION_ERROR(-20669,'CANNOT BORROW BOOKS FROM LONDON STORE DURING MONTH DECEMBER');
END IF;
END;
/
The trigger is not created and have errors please cloud give me correct trigger for this??
Error i have experiencing
Errors: TRIGGER BORROW_TRIGGER
Line/Col: 3/5 PLS-00049: bad bind variable 'NEW.LOC_ID'

Answer 1

Since you only need to perform the check when b_date is in December, it's more efficient to add this as a when condition at the top of the trigger. This also simplifies the trigger logic.
create or replace trigger borrow_check_trg
before insert on borrow
for each row
when (to_char(new.b_date,'MM') = '12')
declare
l_loc_id copy.loc_id%type;
begin
select c.loc_id into l_loc_id
from copy c
where c.copy_id = :new.copy_id;
if l_loc_id = 'LC0001' then
raise_application_error(-20669, 'Books cannot be borrowed from the London store during December');
end if;
end;

Answer 2

You need to query the COPY table to get the field you need:
CREATE OR REPLACE TRIGGER BORROW_BI
BEFORE INSERT ON BORROW
FOR EACH ROW
DECLARE
strLoc_id COPY.LOC_ID%TYPE;
BEGIN
SELECT LOC_ID
INTO strLoc_id
FROM DUAL
LEFT OUTER JOIN COPY c
ON c.COPY_ID = :NEW.COPY_ID;
IF TO_CHAR(TO_DATE(:NEW.B_Date, 'DD-MMM-YYYY'), 'MMM') = 'DEC' AND
strLoc_id = 'LC0001'
THEN
RAISE_APPLICATION_ERROR(-20669,'CANNOT BORROW BOOKS FROM LONDON STORE DURING MONTH DECEMBER');
END IF;
END BORROW_BI;

Answer 3

You can check out the existence by using a Select statement with COUNT aggregation from the other table(copy) through use of common column (copy_id) among tables such as
CREATE OR REPLACE TRIGGER Trg_Borrow_Trigger_BI
BEFORE INSERT ON borrow
FOR EACH ROW
DECLARE
v_exists INT;
BEGIN
SELECT COUNT(*)
INTO v_exists
FROM copy
WHERE copy_id = :NEW.copy_id
AND loc_id = 'LC0001'
AND TO_CHAR( :NEW.b_Date, 'MM' ) = '12';
IF v_exists > 0 THEN
RAISE_APPLICATION_ERROR(-20669,
'CANNOT BORROW BOOKS FROM LONDON STORE DURING MONTH DECEMBER');
END IF;
END;
/
where
TO_DATE() conversion is superfluous
to prepend loc_id with :NEW is not possible, since the trigger is created for
the table borrow has not this column, while copy table has.