Seleziona un cliente specifico in MySQL?

Preferisco l'approccio SUM-CASE:

SELECT x.name
FROM
(SELECT c.name, SUM(CASE WHEN b.paid_out THEN 0 ELSE 1 END) all_paid
FROM customers c JOIN living_places l ON c.ID = l.ID
JOIN policies p ON l.code = p.code_living_place
JOIN bills b ON p.code_policy = b.code_policy
GROUP BY c.name) x
WHERE x.all_paid = 0;

Forse potresti evitare il SELECT annidato usando anche la clausola HAVING qui...