Query SQL to subtract two fields

Question

My query is that I have two tables, one called sec_users with the following fields:

pk_user, name, dias_disponibles

And anothe

Answer 1

You want to subtract the sum of n_diassolicitados from the dias_disponibles. So don't join the table solicitud, but the aggregation query:

SELECT 
  u.pk_user, 
  u.dias_disponibles - COALESCE(s.dias_solicitados, 0) AS dias_libres 
FROM sec_users u
LEFT JOIN 
(
  SELECT fk_empleado, SUM(n_diassolicitados) AS dias_solicitados
  FROM solicitud 
  GROUP BY fk_empleado
) s on s.fk_empleado = u.pk_user;