select

[Libras]

Quita ese not in y usa not exists, es mas rapido, ademas quita ese order by en el subquery, no te sirve de mucho

Código MySQL:

select * from `users_alerts` 
where 
`users_alerts`.id not in
(
SELECT ua.id as vinculo FROM `users_alerts` as ua
join `contratos` as co on ua.id=co.user_id
where `users_alerts`.id=ua.id
group by ua.email
)