I got the task to find the cluster number of a customer, based on where he is living.
you can look in the address table directly and join it with the cluster table:
SELECT c.number FROM address AS a INNER JOIN cluster AS c ON a.cluster = c.id WHERE a.street = 'Teststraße' AND a.street_number = 16
if you want to have the information about all customers, join it with this table as well and remove the where clause:
SELECT * FROM customer INNER JOIN address ON customer.street = address.street AND customer.street_number = address.street_number INNER JOIN cluster ON address.cluster = cluster.id