Voici la requête. Elle regroupe les commandes par client et ne garde que ceux dont la dernière commande date de plus de 90 jours.

SELECT c.id, c.nom, c.email,
       MAX(o.date) AS derniere_commande,
       SUM(o.montant) AS total_depense
FROM clients c
JOIN commandes o ON o.client_id = c.id
GROUP BY c.id, c.nom, c.email
HAVING MAX(o.date) < CURRENT_DATE - INTERVAL '90 days'
ORDER BY derniere_commande;