Your first bracket is wrong:
(JOB(Active,Variable.CUSTOMER_ID)}
should be:
{JOB(Active,Variable.CUSTOMER_ID)}
Generally speaking I find it easier to declare variables, and use those variables in the query. An additional advantage is that you avoid implicit type conversion. In your case the MySql engine has to convert a string to int, and this can cause performance issues in some cases. This is how it would look like in SQL Server (I don't know the syntax for MySql)
DECLARE @customerId AS INT = CAST({JOB(Active,Variable.CUSTOMER_ID)} AS INT);
select *
from business.customers
where customer_id = @customerId;