sumar de otra tabla teniendo group by

[jurena]

No he probado nada con datos. Es una propuesta al vuelo.

Código MySQL:

SELECT t2.age, SUM(t2.amount) FROM  (SELECT
        CASE WHEN age >= 45 THEN 45
        WHEN age >= 31 THEN 31
        WHEN age >= 26 THEN 26
        WHEN age >= 18 THEN 18
        ELSE 0 END as age_groups, t1.amount
FROM 
(
SELECT FLOOR(DATEDIFF(CURRENT_DATE,`dateOfBirth`)/365) as age, vc.amount FROM users INNER JOIN ventasporcliente vc ON users.idClient = vc.idClient
) t1) t2 GROUP BY t2.age