select  cust_email
from OrderItems join Orders using (order_num)
join Customers using(cust_id)
where prod_id='BR01'
;

from t1 join t2 using (x)

优于

from t1 join t2 on t1.x = t2.x

优于

from t1,t2 where t1.x = t2.x ;

注意 using需要后面跟括号（即使单列）