PL/SQL Invalid Identifier - oracle

Question

While creating a trigger on the 'emprunter' table, I'm trying to compare a value coming from another table 'exemplaire.numexemplaire' which is supposed to be an INTEGER. But I keep getting the same errors which is:
Error(4,7): PL/SQL: SQL Statement ignored
Error(7,15): PL/SQL: ORA-00904: "EMPRUNTER"."NUMEXEMPLAIRE":
invalid identifier
How can I do to retrieve the value of a field coming from another table (exemplaire.numexemplaire)?
CREATE OR REPLACE TRIGGER BIEmprunter
BEFORE INSERT OR UPDATE OF numexemplaire ON emprunter
FOR EACH ROW
DECLARE
livreEmpruntable INTEGER;
BEGIN
SELECT exemplaire.empruntable
INTO livreEmpruntable
FROM exemplaire
WHERE emprunter.numexemplaire = exemplaire.numexemplaire;
IF livreEmpruntable != 1 THEN
raise_application_error(-20000, 'exemplaire non empruntable');
END if;
END;
UPDATE 1
Thanks for the answer but I keep getting this error whule trying to test the trigger...
SQL Error: ORA-04098: trigger 'EQUIPE10.ABONNEMENTPASAJOUR' is invalid and failed re-validation
04098. 00000 - "trigger '%s.%s' is invalid and failed re-validation"
*Cause: A trigger was attempted to be retrieved for execution and was
found to be invalid. This also means that compilation/authorization
failed for the trigger.
*Action: Options are to resolve the compilation/authorization errors,
disable the trigger, or drop the trigger.
Update 2
Thanks again for the answer, the trigger can compile now. But when now I'm trying to make it work when inserting value but I keep getting the error because there is no data yet...
INSERT INTO emprunter
VALUES (2, 1, 18, '17-02-01', null);
Error report -
SQL Error: ORA-01403: no data found
ORA-06512: at "EQUIPE10.BIEMPRUNTER", line 4
ORA-04088: error during execution of trigger 'EQUIPE10.BIEMPRUNTER'
01403. 00000 - "no data found"
*Cause: No data was found from the objects.
*Action: There was no data from the objects which may be due to end of fetch.

Answer 1

You want to join the two tables:
CREATE OR REPLACE TRIGGER BIEmprunter
BEFORE INSERT OR UPDATE OF numexemplaire ON emprunter
FOR EACH ROW
DECLARE
livreEmpruntable INTEGER;
BEGIN
SELECT exemplaire.empruntable
INTO livreEmpruntable
FROM exemplaire
join emprunter
ON emprunter.numexemplaire = exemplaire.numexemplaire;
IF livreEmpruntable != 1 THEN
raise_application_error(-20000, 'exemplaire non empruntable');
END if;
END;
Also, the above join query can potentially return multiple rows and the call will result in exception (uncaught as of now)
ORA-01422: exact fetch returns more than requested number of rows

Answer 2

I guess you want to use :new to access the record that triggered the trigger:
CREATE OR REPLACE TRIGGER BIEmprunter
BEFORE INSERT OR UPDATE OF numexemplaire ON emprunter
FOR EACH ROW
DECLARE
livreEmpruntable INTEGER;
BEGIN
SELECT exemplaire.empruntable
INTO livreEmpruntable
FROM exemplaire
WHERE :new.numexemplaire = exemplaire.numexemplaire;
IF livreEmpruntable != 1 THEN
raise_application_error(-20000, 'exemplaire non empruntable');
END if;
END;