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!

Answer 2

The user1 creates another request for 2 days of sick leave, those 2 days of sick leave do not count as requested days. How can I show those two days requested, using the following code:

SELECT u.pk_user, 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

Answer 3

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;