¿Como soluciono más de una fila en una Subquery dentro de unTrigger?

Tengo un ejercicio que me pide. Crear un trigger que al borrar una región borre todos los datos relacionados en el resto de tablas, es decir COUNTRIES, DEPARTMENTS, LOCATIONS y EMPLOYEES.

el trigger que he creado es el siguiente:

CREATE TRIGGER EJ7 BEFORE DELETE ON REGIONS FOR EACH ROWBEGIN    DECLARE PRI_REGION FLOAT;    DECLARE PRI_COUNTRIES CHAR(2);    DECLARE PRI_LOCATIONS FLOAT;    DECLARE PRI_DEPARTMENTS FLOAT;    DECLARE PRI_EMPLOYEES FLOAT;    SET PRI_REGION=OLD.REGION_ID;    SET PRI_COUNTRIES=(SELECT COUNTRY_ID FROM COUNTRIES WHERE REGION_ID=PRI_REGION);    SET PRI_LOCATIONS=(SELECT LOCATION_ID FROM LOCATIONS WHERE COUNTRY_ID=PRI_COUNTRIES);    SET PRI_DEPARTMENTS=(SELECT DEPARTMENT_ID FROM DEPARTMENTS WHERE LOCATION_ID=PRI_LOCATIONS);    SET PRI_EMPLOYEES=(SELECT EMPLOYEE_ID FROM EMPLOYEES WHERE DEPARTMENT_ID=PRI_DEPARTMENTS);    DELETE FROM EMPLOYEES WHERE EMPLOYEE_ID=PRI_EMPLOYEES;    DELETE FROM DEPARTMENTS WHERE DEPARTMENT_ID=PRI_DEPARTMENTS;    DELETE FROM LOCATIONS WHERE LOCATION_ID=PRI_LOCATIONS;    DELETE FROM COUNTRIES WHERE COUNTRY_ID=PRI_COUNTRIES;END:

Y al ejecutar un DELETE FROM REGIONS WHERE REGION_ID=1:ERROR 1242 (21000): Subquery returns more than 1 row

Como podría solucionar mi problema.