URGENT: ORACLE TRIGGER: update a field (field+1 and field -1) when inserting and deleting : This is a bit urgent doubt I have for my DBMS on the next 1.30h. 

I need to make a trigger that when inserting or deleting an employee (EMPLOYERS TABLE), the attribute EMPTOTAL from table SHOPS UPDATE. 

EMPLOYERS table has a field (and Foreig Key) called SHOP that references the SHOPS table. 

I know it should be something similar to this, but I don't have any example that involves more than 1 table on my exercises. 


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CREATE OR REPLACE TRIGGER UPD_EMPTOTAL BEFORE INSERT OR DELETE ON EMPLOYERS FOR EACH ROW
DECLARE
BEGIN
IF INSERTING THEN
UPDATE TIENDAS SET EMPTOTAL=EMPTOTAL+1;
ELSIF DELETING THEN
UPDATE TIENDAS SET EMPTOTAL=EMPTOTAL-1;
END IF;

END;

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Thank you in advance!