Subquery problems

Hello,

I'm having another "problem", I have a function that declares 12
variable, one per month and each them execute a select like bellow:
*DIV_MES01 := (select count(distinct production_date) from production
where extract(month from production_date) = '01' and extract(year from
production_date) = EXTRACT(YEAR FROM current_date));

*Then, I need to check if the variable is equal 0:
*IF DIV_MES01 = 0 THEN
DIV_MES01 := 1;
END IF;

*Finally, I perform the following query:

*SELECT cast(((sum(A.qty_employees_total)
-(sum(A.qty_absence)
-sum(A.qty_vacation)
-sum(A.qty_diseased)
-sum(A.qty_indirect)
-sum(A.qty_transferred))
+sum(A.qty_received))/DIV_MES01) AS integer),
C.id_production_area,
cast('01' as text) AS mes
FROM head_count A, machine B, machine_type C
WHERE EXTRACT(YEAR FROM head_count_date) = EXTRACT(YEAR FROM
current_date)
AND EXTRACT(MONTH FROM head_count_date) = '01'
AND A.id_machine = B.id_machine
AND B.id_machine_type = C.id_machine_type
GROUP BY C.id_production_area, B.id_machine_type

*Doing it, I need to perform 12 querys united by "UNION", what I want to
do is unify it in only one query, I tryed with the query bellow:

*SELECT date_trunc('month', A.head_count_date)::date as head_date,
cast(((sum(A.qty_employees_total)
-(sum(A.qty_absence)
-sum(A.qty_vacation)
-sum(A.qty_diseased)
-sum(A.qty_indirect)
-sum(A.qty_transferred))
+sum(A.qty_received))/(select count(distinct production_date)
from production
where extract(month from
production_date) = EXTRACT(MONTH FROM date_trunc('month',
A.head_count_date)::date)
and extract(year from
production_date) = EXTRACT(YEAR FROM current_date))) AS integer),
C.id_production_area
FROM head_count A, machine B, machine_type C
WHERE date_trunc('month', A.head_count_date)::date BETWEEN
date_trunc('month', current_date - (EXTRACT(MONTH FROM
current_date)-1) * interval '1 month')::date
AND date_trunc('month', current_date)::date
AND A.id_machine = B.id_machine
AND B.id_machine_type = C.id_machine_type
GROUP BY C.id_production_area, B.id_machine_type, head_count_date,head_date
ORDER BY id_production_area, head_count_date,head_date DESC

*But the results aren't what I want.
What I trying to do is possible?

I appreciate any help.
Thanks