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

Use SUM aggregating function:

SELECT u.pk_user, sum(u.dias_disponibles) - sum(s.n_diassolicitados) AS dias_libres 
FROM   sec_users u
LEFT JOIN solicitud s 
on s.fk_empleado = u.pk_user
GROUP BY u.pk_user

...and always follow tip 1 from comments!